If you guys are ready to get hitched then getting the letter from her parents isn't an issue isn't it? Unless of course you are planning to elope? 

And if she's only 20, I would definitely not even consider marriage (unless she's pregnant, in which case, you've both proven yourself to be too irresponsible to get married).
I should not have taken the bait. I wish the admin would do away with anonymous posting.  

Unless rules have changed, you need a letter from the parents to apply for a spouse VISA from a consulate or an embassy. You will need the Hukou (here again, usually in the parent's hands) to apply for residency.

Why everything from the parents? Because in China, choices aren't yours to make, you always belong to someone else. Take it as a "warm tip".

A woman needs to be older than 22 in order to marry legally.

We didn't need that, but my wife was 26 when we got married.... oh, I think her mum was with us at the marriage office but only first attempt when they said one of our documents wasn't OK, so i got to take a bus to the embassy for a stamp and then we tried the next day again. 

Nope, not anymore - you just need the hukou now. If you think about it, the hukou usually is a "signed letter," since it's usually in the hands of the head of the household, and you normally can't duplicate it, so the whole thing is redundant. If you can't get the hukou, sue for it, or apply for another one saying you "lost" it.

Married my wife at 22.

Beep boop?