To study the competition mechanism of trees,and optimize it,s structure and adjust thinning technology to Wangqing Quercus monyolica secondary forest, this article builds the crown model of the Quercus monyolica free-tree and the influential factor and the circle of influence symbiotic mode judgment factor. Based on the 108 of the Quercus monyolica advantages trees data,using regression analysis,adjusting the coefficient associated with the construction of crown and DBH model of Quercus mongolica advantage trees,the result for CW =0. 1094 +0. 2998 * D. Based on the definition of forest competition,the influential factor was used to determine the competition wood of the object wood. The symbiotic mode between the influence circle of objects wood and the influence circle of competition wood was divided into four types,the determine factors affecting the symbiotic circle FSI was used to judge the symbiotic mode and calculate the various symbiotic mode crown overlapping area. Based on different models, the corresponding competitive pressure index CSI measurement model was used to calculate the value. And the case was studied,the CSI value of the sample tree was 439.