To understand the effects of different forests on soil organic matter decomposition, enzyme activities involved in the transformations of soil carbon, nitrogen and phosphorus under the forests of Quercus variabis, Pinus tabulaeformis and Quercus variabis mixed with Pinus tabulaeformis were measured by using the international methods about the measurement of soil enzyme activity in lower mountain area, Beijing. The activities of cellobiase, α-glucosidase, β-glucosidase and urease under Quercus variabis forest were significantly higher than those under the mixed forest and Pinus tabulaeformis forest; whereas, the activities of peroxidase and phosphatase under Pinus tabulaeformis forest were significantly higher than those under Quercus variabis forest and mixed forest in 0-5 cm soil layer. Urease activity under Quercus variabis forest was significantly higher than that under mixed forest and Pinus tabulaeformis forest. However, the differences of other enzyme activities were not significant under different forests in 5-10 cm soil layer. Soil pH had a significant negative correlation with soil peroxidase and phosphatase activities, whereas the activities of cellobiase, α-glucosidase, β-glucosidase, phenol oxidase, chitinase, protease and urease were significantly positive correlated with the contents of soil organic carbon, dissolved organic carbon, soil total nitrogen and the dissolved organic nitrogen. The mixed forest of Quercus variabis and Pinus tabulaeformis could regulate soil enzyme activities and soil organic matter decomposition.