In this paper, the regression equation between chlorophyll-a (Chla) and some environmental factors was calculate by multiple stepwise regression, based on the monthly monitoring data in the Qiandao Lake from Jan. 1999 to Dec. 2000, and modified Carlson trophic state index for all the sampling sites was also analyzed. The results indicated that the spatial and temporal distribution of chlorophyll a in the sampling sites were varied strongly, the concentration of chlorophyll a in the Xin'an river changed from 50 mg.m-3 to 30 mg.m-3, but that in the lake was under 5 mg.m-3. The modified Carlson trophic state index (TSIM) for the sampling sites in southeast of the lake was under 37 (belonging to olligotrophic), and the others were under 53 (belonging to mesotrophic). In different sampling sites, the significant environmental affecting factors for chlorophyll a were different. However, the relationships between chlorophyll a and water temperature and total phosphorus were positive, and that between chlorophyll a and diaphaneity was negative.