From the Chinese web, this is where you can donate used clothing in Beijing.  The clothes must be washed prior to donating them and the charities will not accept underclothes, socks and hats.

Beijing donated old clothes donations for disaster relief affairs management center in Beijing Jinsong seven districts 010-67757271 725 , Haidian District, Haidian District, received social contributions workstation Xiyuan playground 010-62625347 No.15 Dongcheng District social contributions receiving workstation, Dongcheng District, No. 13 32 010 -64,001,306 Xicheng District community donor receives workstation, Takai Hutong, Xicheng District, Chongwen District, the 16th 010-62274355 social contributions receiving workstation Peixin Street, Xuanwu District, social contributions in the 2nd 010-67162676, Chongwen District, receiving workstation the Xuanwu District Chong efficiency alley on the 18th 010-63556360 Chaoyang District Social donor receives workstation Road, Chaoyang District, homes, Building 6, Deputy House 010-65024588 Fengtai District social contributions to receive the workstation North Street on the 9th 010-68312023, Fengtai District Shijingshan District Social donor receives three workstation Shijingshan District octagonal North 010-88913057 clothing donation requirements : donate clothing to ensure clean, sanitary, and took seven or more gently used, no damage, it is best to clean, disinfect, used underwear and shoes and hats can not donate.

I leave unwanted clothes in the area where people leave recyclables for the collectors. I like to think someone there can use them.