Abstract: The changes of chlorophyll fluorescence parameters and free water and bound water of three typical heteromorphic leaves of Populus euphratica, both in young trees and old trees, were investigated. The results showed that the values of F0, Fm, Fv/Fm and Fv/F0 of lanceolate leaves were the least in young trees, while those of Fm,Fv/F0 and Fv/Fm of ovate leaves were the least in old trees. Compared with the young trees, the free water content of heteromorphic leaves of old trees increased significantly by 78.94% in lanceolate leaves, and in the leaves of ovate and serrated broad-oval,it increased by 10.60% and 10.99% respectively. The analysis of correlations showed that the free water content was significantly positive related with Fv/Fm and Fv/F0 in young trees，while the relationship of total water content with Fv/Fm and Fv/F0 was positive in old trees.