Abstract: Abstract：The Daya Bay is a big bay in eastern of Guangdong, adjoining Shenzhen and Hongkong. The current state and tendency of its environment attract attention of many researchers. The heavy metallic elements Hg, As, Cd, Cu, Pb, Cr and Zn in 45 samples of surface sediments, 85 samples from one drilling hole and 2 cores in the Daya Bay were analysed. Based on the distribution of heavy metals, the substrate environment was quantitatively assessed. The results are as follows. (1) The pollution indices Pi of Hg, As and Cu are all <1.0, suggesting that the entire bay is not polluted by these factors. (2) Some pollution indexes of Cd, Zn, Pb and Cr are > 1.0, suggesting that the bay is polluted by these factors to different degrees, of which the pollution index of Pb changes from 0.83 to 2.25, indicating low to moderate pollution and on the whole slight pollution; the index of Zn changes greatly from 0.25 to 2.63, generally showing slight pollution; the index changes from 0.50 to 1.78, showing low to slight pollution; the index of Cr changes from 0.49 to 1.90, showing low to slight pollution. (3) The synthetic pollution index (SQI) varies from 1.08 to 2.21, with a mean of 1.37, suggesting that the area is polluted by heavy metals to different degrees, generally showing slight pollution. The most serious pollution appears in the Fanhe Harbor, with most SQI values＞1.5, and the second most serious pollution appears in the sea area near the Central islands. The mouth of the bay and the sea area off the coast from Aotou to Xiayong are less polluted. The contents of heavy metals on the surface have no marked tendency of increase, as shown from the distribution of the heavy metal contents of samples from the drilling hole and cores.