if its your house... I guess you are the boss...

confront her ..constantly nag her... you cant just let someone doing anything disgusting only bcoz she is a senior citizen or your MIL...

Tell her not to do it.. either the spitting will stop or she will move away..

I am not married but my  partner's parents already stayed in our home as they are living in Northern China and we are in Guangzhou, they won't come for only one day. They are nice people and have absolutely no problems with the fact that their only daughter is dating a foreigner, but as with every peasant background families they have nasty habits.

Don't let her do it, the first time I caught her mother spitting I said (in Chinese): "this is my home, if you stay here you abide by my standards, next time you spit here I kick you out." she never did it again for the month they stayed with us, and never did it again at all in front of me.

This is how things work with Chinese people, if you don't say anything they will push the limits again and again to see how much they can get away with. If you are strict and don't let anything pass from the beginning they will respect you.

Tell her to stop. You should speak to her about it and if your Chinese is not good enough put pressure on your wife to make her. If this fails build a barn for her to live in.

Tell her to stop, and explain that it's disgusting to you, that a clean home means a lot to you. Say it everyday, every time. You have small, small chances to see improvements.

I had almost the same problem

Last year I was taking care of a parrot while it's owner was on extended holiday. I got sick with a bad cough for a week and the bird started making these gross hacking/cough sounds. So I had this big stupid (though to be fair very lovable) bird coughing all the time, people would ask "is that thing dying?" and I'd have to explain that the bird was a fraud and just faking illness to get sympathy head scratches/pumpkin seed treats. It was really a terrible, humiliating situation.

The owner said the best thing to do is get a recording of more pleasant sounds/words and play it while you're at work. With luck the new ones will replace the bad hacking sounds.

You can try this solution as well.

Today my MIL confronted me about why I had boiled some water yesterday that I didn't use. OK, so the water boiler in the kitchen is full and Mrs Sherlock Holmes has deduced that I have boiled 1,5 liters of water without using it. She is in fact correct, and she is right that this is a massive waste of electricity. However. The women is incapable of closing doors. We have ACs on and windows open. I am pretty sure the waste from this equates, in CNY, a lot more waste each day than the water boiling.

However. ACs were not present in her childhood home, but water boilers were. She cannot be reprogrammed from the fact that a "water boiler uses expensive electricity" to "and AC uses WAY more power than a water boiler"

In a curious side note. She never closes the lid on said water boiler, meaning the heating element will be on for a lot longer for the heat sensitive switch in the handle to click off. 

The morale of the story. Throw her out. You will have kids who spit and crap everywhere. 

I'd give a more nuanced answer to a non anonymous post. 

Every time you hear her spit put on rubber gloves, a surgical mask, safety glasses and follow her around cleaning up her mess with powerful smelling bleach and a scrubbing brush. She might get the point then.

If all else fail, buy her a spittoon and tie it around her neck. Maybe then she will get the message. Nasty habit. 

Spit on her.

That is fucking disgusting... she needs her face to be honored, though... so you can't do anything but come honor face.

For any negatives this website might have, it cannot out weigh the huge positive being that this website constantly makes me appreciate my wife more. She has done an admiral job of shielding me from any of her families nasty traits. If they even have any other than saying some dumb shit once in a while. They have never actually done anything to frustrate me. I am very lucky. 

Having a bad MIL day, so here is another sing at "the MILs"

Spitting, sure it is disgusting, it is very obvious, but this is most likely not the biggest "threat" to your children. Spitting is easy to correct as a parent...... But what about all the shit the MIL tells the kids.
Don't drink cold water, thunder is dangerous. Endless crap that will make your children as disconnected from the real world as a kid who've grown up on a movie set for a movie about Santa. Superstition upon superstition, piled up in the kids formative years. Your kids will get a "no can do" attitude because there is always some superstitious reason not to do things.

Today I just told my wife about Fan Death
http://en.wikipedia.org/wiki/Fan_death
How f'ed up is that right.... everything your MIL believes is equally f'ed up.
After many years together, my wife is now able to spot bull like this, at least most often. if I told my MIL about it, she'd not be able to figure out if she is in danger. She might deduce she isn't as Chinese bodies are different from Koreans (that one is for you Pogger34)

My MIL lives faaaaaar away . But she's cool anyway haha.

Get yourself a T shirt printed 'No spitting on the floor' if that doesn't work think about rubbing her nose in it. Filthy habit.