The researcher summarized the principles, merits and demerits, applicable ranges of different methods studying plant water use efficiency (WUE). The results indicated that the stable carbon method will be used on a wide range because it can simplify the measuring process, make the data more accurate and can also overcome the difficulty that other methods can not measure plant population in different areas at the same time. However, the measurement cost of the stable carbon method is higher. The substituting index method will continue to be a main direction of studying plant water use efficiency due to its small cost and easy measurement, thus this method has optimistic prospect. In general, the method combing the conventional direct measurement with gas exchange method is the most applied in China nowadays, and they all calculate the WUE through measuring the leaf gas exchange. So, the research in the future must focus on the investigation into the seasonal change of δ¹³C and WUE of riparian forest in extreme arid areas and their despondence to the change of environmental factors by multi-methods at multi-space-time levels.