With simulation test, this paper studied the effects of Hg on the activities of urease, invertase and neutral phosphotase in four soils. The results showed that Hg inhibited soil urease and invertase activities markedly, but its inhibitory effect differed with test soils. There was a significant logarithmic correlation between the concentration of HgCl2 and the activities of these two enzymes (P < 0.05). In test soils, the ED50 of urease activity was 87.99, 5.47, 24.05 and 19.88 mg x kg(-1), and that of invertase activity was 76.68, 727.49, 236.52 and 316.59 mg x kg(-1), respectively. Urease was more sensitive than invertase to Hg contamination, while organic matter had a protective effect on soil enzymes. Soil neutral phosphatase was not sensitive to Hg contamination, except that it was significantly activated by Hg in the meadow brown soil applied with plenty of organic fertilizer.