Beijing social security is levied by local taxes.

Since August 2008, 23 of the 36 provinces and cities in China have been paid social security fees by local tax authorities: Hebei, Inner Mongolia, Liaoning, Heilongjiang, Jiangsu, Zhejiang, Anhui, Fujian, Hubei, Hunan, Guangdong, Hainan, Chongqing, Yunnan, Shaanxi, Gansu, Ningxia, Dalian, Ningbo, Xiamen, Qinghai, Beijing and Henan.

Further reading: How to buy insurance, which is good, and teach you how to avoid these "pits" of insurance.