Six types of 25 years stand density(3 400 plants/hm²,2 900 plants/hm²,2 600 plants/hm²,2 300 plants/hm²,2 150 plants/hm² and 1 750 plants/hm²) were selected,and the law of importance values,species richness,species diversity index,evenness index with stand density were analyzed to study the relationship between stand density and plant diversity of under growth of Picea crassifolia plantation in Baoku Forest farm at Datong County of Qinghai Province.The aim of this paper was to verify an appropriate stand density that could improve plants diversity and maintain the stability of plantation ecosystem.The result showed that:(1)There were 60 vascular plants and one nonvascular plant in 45 families and 29 genera of under growth in P.crassifolia in the study area.(2)Stand density had a significant influence on dominant plants in shrub layer,but the influence was not obvious in herb layer;(3)There was a positive correlation between stand diversity and species richness index,Shannon-Wiener index and Alatalo index in shrub layer,and the correlation coefficients were 0.688,0.937,0.762,respectively.With respect to herb layer,the correlation between them was also obvious,and the correlation coefficients were 0.679,0.870,0.505,respectively.(4)As stand density increased,plants diversity decreased firstly,then increased and finally decreased.The research suggested that the response of plant diversity of understory vegetation to stand density was non-synchronous.When density arrived at 2 600 hm²,the understory plant diversity reached the maximum,which indicated that the density was more appropriate for growth and development of understory vegetation,and can contribute to an increase in forest diversity.