Abstract: The Platycladus orientalis plantation is one of the major non-commercial plantations in Beijing mountainous areas.Due to the generally high stand density, the forests could not grow well.There is very little vegetation undergrowth,the biodiversity is degrading,and the general quality of the stand is in poor condition.The ecological functions of the plantations hence cannot be fully activated.Tending,as the fundamental method of forest management,has significant effects in improving the general quality of plantations,recovering the undergrowth diversity and reconstructing the deteriorated ecosystem.In order to investigate the dynamic variance of the undergrowth and to provide reference for reasonable tending,in 2004,41 sample plots in the low hill sunny slope with thin soil,sunny slope with thick soil and half sunny slope with thick soil,were selected in Beijing mountainous areas and light,medium and heavy tending on each sample plots were conducted,respectively.In the summers of 2005 and 2006,changes of the undergrowth diversity were investigated and analyzed.The results showed:1) in the 1st and 2nd tending years,the species and quantity of undergrowth,Simpson index(D),Shannon-Wiener biodiversity index(H') and the total amount of the undergrowth biomass all increased along with enhancing tending intensity;and the distribution of vertical structure of the community became more significantly as the tending intensity was enhanced;2) after two years of heavy tending,the shrub species increased in the sunny slope with thin soil,sunny slope with thick soil and half sunny slope with thick soil by 4,5 and 5,and these numbers for the herb species increased by 13,10 and 8 respectively.Besides, the heavy tending has an effect in improving the undergrowth's turning to xero-mesophytes and meso-xerophytes;3) after the medium and heavy tending,the undergrowth biomass can reach more than 5 t/hm².Among the short-term effects,the medium and heavy tending are more favorable to the increase of plant diversity.