Abstract: Water use efficiency (WUE) is an important indicator of evaluating irrigation efficiency. However,errors associated with WUE in evaluating irrigation efficiency of regions with different climate have been neglected. Based on the concept of Potential Water Use Efficiency (PWUE) and meteorological data,this study fills this gap by calculating the PWUE of wheat,maize,and rice of China in 2014 as well as evaluating the effect of climate difference on WUE. We put forward and examine concepts and calculation method of Relative Water Use Efficiency (RWUE) and Theoretical Agricultural Water Saving Potential (TAWSP).Our results show that standard deviations of PWUE of main grain crops are between 0.49 and 1.01 kg/m³,and the spatial variance of PWUE of most crops was larger than their actual WUE. The average RWUE of main crops is 50.7% and their TAWSP are between 884.8 and 4 064.5 m³/hm². More importantly,the index of WUE and traditional agricultural water saving potential may overestimate or underestimate the water use efficiency and water saving potential of crops. In addition,we find that the impact of climate difference should not be neglected in comparing WUE in different regions. The indexes of RWUE and TAWSP are more reasonable as they can take into account the regional climate difference.