Sludge yield can be calculated by the empirical sludge yield and sewage treatment volume. Due to the change of influent quality and the difference of operation control, the sludge yield of the same WWTP will be different with the seasonal variation. By the investigation, survey and study of four typical wastewater treatment plants (WWTP) in in Hangzhou, the rule of sludge yield with the seasonal variation is analyzed at first, and then the specific influencing factors on the sludge yield are further explored from three aspects of influent COD concentration, influent SS concentration and operating water temperature. Research results show that the sludge yield in spring of WWTPs is higher than in the other three seasons, and the sludge yields in summer and autumn are relatively lower. There is a great correlation between sludge yield and influent COD. The sludge yield increases with the increment of COD value. When the influent SS concentration is high, the sludge yield is relatively higher.