In terms of photosynthetic efficiency,which demonstrates the space-time function of plant,niche width of B. Platyphylla population was described.First,on the basis of net photosynthetic amount in certain time,the niche width of a birch was calculated.Then,the realized niche and rudimental niche of B. Platyphylla seedlings,young trees and adult trees were discussed.Finally, the dynamic of niche under special conditions was studied.