In order to investigate the influences of thinning on soil quality in young Pinus tabulaeformis plantations and to provide a reference for reasonable thinning,a thinning experiment was conducted in the 18-year-old Pinus tabulaeformis plantation in lower hilly lands of Yingpan and Yanqing County,Beijing City.Four different thinning treatments(0,35.7%,49.2%,and 64.2% thinned) were carried out in the stands of 3130 tree/hm2 initial density.In 2005,the 3rd year after thinning,soil physical properties,chemical properties,and soil enzyme activities in the four sample plots were studied.Results showed that thinning has significant effects on soil quality in the 18-year-old Pinus tabulaeformis plantation.The thinning would improve capillary spaces and water-holding capacities.Along with the density decreasing,soil organic matter,available K,and available P increased significantly.The catalase,polyphenol oxidase,urease,and alkaline phosphatase activities increased along with the thinning increasing.Results from principle component analysis revealed that soil catalase,urease,and available P could be regarded as indicators to assess soil quality affected by thinning.In the plantations of decreased reserve density,the integrated fertility indexes of soil quality properties were-1.061,0.032,0.347,and 0.682,respectively,among which the 64.2% thinning intensity was the best.