In order to understand the ecological characteristics of phytoplankton’s dominant species and their relationships with environmental factors in summer and autumn in Dianshan Lake, a survey was conducted at 13 sampling sites from June to November 2012. The density and dominance and niche breadth were calculated in this study. The phytoplankton dominant species in the Dianshan Lake was composed of 17 species belonging to4 phylum and 12 genera. The density of phytoplankton’s dominant species which appeared every month and the total densityof phytoplankton were analyzed with pearson correlation,and we screened out that  Chlorella vulgaris, Chroomonas acuta, Melosira ambigua and  Cryptomonas rostrata were the main dominant species.The result of grey relationship analysis suggested that different phytoplankton density varied with the change of environmental factors, and the factors which seriously influenced each species were different in summer and autumn. In summer, transparency was the key factor that influenced the density of  Chlorella vulgaris, Melosira ambigua  and Cryptomonas rostrata. Dissolve oxygen wasthe key factor that influenced the density of  Chroomonas acuta.  In autumn, total phosphorus was the key factor that influenced the density of Chlorella vulgaris. Dissolve oxygenwas the key factor that influenced the density of Chroomonas acuta  and Cryptomonas rostrata. Total nitrogen was the key factor that influenced the density of  Melosira ambigua.