Abstract: Selecting Leuce Duby and Aigeiros Duby seedlings as materials, this paper analyzed the responses of gas exchange parameters(Pn、Tr、WUE) and fluorescence parameters(Fv/Fm、Fmv/Fms) of five breeds of aspen and Crossbreed to Drought Stress, and comprehensively evaluated the drought-resistant characteristics of five breeds. It was found that the net photosynthetic rate and transpiration rate of five breeds obviously descended with the descend of leaf water potentials, at the initial stage of water stress, the net photosynthetic rate of Populus nigra and P. alba showed a obvious increasing trend. Before midium drought stressing,the water using efficiency of all the five breeds all showed the phenomenon of increasing with the descending of leaf water potential, the drought-resistance characteristics maybe a regular reflection of these breeds. Fluorescence parameter Fv/Fm of different breed wholly gradually descended with the descending of leaf water potential, the value of leaf water potential at the sudden descending point of Fv/Fm might become a new index to compare the drought-resistant capacity of different breeds. Compared with Fv/Fm, fluorescence parameter Fmv/Fms could directly inflect the practical solar energy capture capacity. While the leaf water potential of seedlings -1.5 MPa, the comprehensive evaluation result of drought-resistant capacity of the five breeds were: transgenic P. nigra carried Bacillus thuringiensis (Bt) gene > P. nigra > P. alba > P.×canescens Smith, P. tremula, which was same to the result while water using efficiency was only adopted, and near to net photosynthesis rate, water using efficiency had proved to be a relatively reliable index.