Abstract: The effects of thinning intensity with four different treatments of 0(CK),40.1%(Ⅰ),51.3%(Ⅱ) and 65.7%(Ⅲ) on the undergrowth were carried out in the 18 years old Pinus tabulaeformis stands with initial density of 3 130 trees/hm² in the lower hilly lands of Yingpan,Yanqing County of Beijing.In 2005,ie three years after thinning,the composition,abundance,diversity,and biomass of the undergrowth in the four plots were investigated and compared.The results showed that both the number and evenness of species predominated by genus and family were enhanced by thinning density.Meanwhile,the life-form spectra were influenced and the percentage of chamaephytes was as the following:Ⅲ<CK<Ⅰ<Ⅱ,opposite to the trend of the phanerophytes.Also,the composition of dominant species of the four undergrowth varied greatly,which of the herb layer responded more than that of the shrub layer.Compared with CK,the abundance of undergrowth was improved,in which the undergrowth with middle thinning was the greatest followed by the slight thinning intensity.Additionally,the biomass of undergrowth was positively related to the thinning intensity.It is concluded that reasonable thinning intensity is benefitial to the development of the undergrowth,and so can be an effective way to the sustainable use of plantations.Generally,when the stand with initial density of 3 130 trees/hm² growing up to 18 years old,the reasonable thinning intensity should be about 51.3%.