Abstract: The effects of physicochemical parameters in the overlying water and sediments on the alkaline phosphatase activity in Erhai Lake were discussed by studying the temporal and spatial variation of this activity from November 2010to August 2011. The results indicated that:① The activity of alkaline phosphatase of the surface sediments changed from 45.63to 144.67μmol/(g·h), and the decreasing order of the activities in different regions were the north, the south and the central. The regions where the exogenous pollutant input had a significant impact had high activity levels of nitrogen, phosphorus and the alkaline phosphatase. ② The alkaline phosphatase activity in the surface sediment showed a obvious seasonal variation in general and its seasonal increasing order were autumn (November), winter (February), spring (May) and summer (August). Its seasonal variation showed significant differences in the western coastal region. ③ The alkaline phosphatase activity had significant positive correlations with concentrations of OM, TN, OP, TP and C/N in the sediments and with concentrations of OP, TN and COD_Mn in the overlying water. The physicochemical parameters in the sediments and overlying water were the internal factors and the exogenous pollutants input were the external factors that are important to affect alkaline phosphatase activity.