Dongfeng Xisha Reservoir, located in the west side of Chongming Island, is the second largest source of drinking water in Shanghai. In order to understand the pollution of heavy metals in surface sediments, the particle size of surface sediment samples collected from 10 sections of the reservoir in 2019 was analyzed, the contents and potential ecological risk of nine heave metal elements were analyzed and evaluated. The results showed that the average content of sand, silt and clay were 46.29%,42.63% and 11.08%, and on the whole, the surface sediment of the reservoir is fine sand sediment. The average contents of Hg, Cd, As, Ni, Pb, Cu, Cr, Zn, Mn were 0.02,0.68,11.30,13.07,18.51,27.73,62.18,82.29,488.66 mg/kg. Only the average contents of Cd and As were 5.23 and 1.24 times the Shanghai soil background value, and the others were below this background value. Pearson correlation coefficient showed that the sources of heavy metals in surface sediments were similar and the particle sizes played an important role in the distribution of Hg, As, Cu, Zn, Cr and Pb; industrial wastewater, agricultural wastewater and domestic sewage were the probable sources by the method of principal component analysis (PCA). The result of Geo-accumulation index showed that Cd was the main pollution element, which was generally at a moderate pollution level, while the other 8 elements were at a clean pollution level. Potential ecological risk assessment showed Cd was the main ecological risk contribution factor, which is at a strong ecological hazard level, and the other 8 elements were at a slight ecological hazard level. The reservoir is generally at a moderate ecological risk level.