Whether plants adapt the local condition limited or not mostly depend on whether they may coordinate the relationship between carbon assimilation and water dissipation or not, that is, plant water use efficiency is the key factor of their surviving. However, at the different scale, the meaning of plant water use efficiency is different, this paper summed up the studying progress of WUE at the leaf level, which it is mostly researched in the recent, and presented following points studying the leaf water use efficiency for the future: for the methods, Foliar carbon isotope is the optimal way measuring plant foliar long water use efficiency at present, and the substitution indices of δ13C will continue to be a prospective direction; For studying contents, we should strengthen the research on δ13C and WUE of the riparian trees in the extreme arid area; Combined the plant ecophysiology, biology with the stable isotope technique, exploring the mechanism of plant foliar water use efficiency, especially reinforcing the studying of laws and interior mechanism of plant foliar long water use efficiency variety by means of the model of double isotope; using multi-means, research on the WUEs through various temporal-spatial scales and conversion.