Abstract: According to the climate data from 1991 to 2000 at Zhangye weather station, using Hargreaves equation and Penman-Monteith equations, the reference crop evapotranspiration(ET₀) was calculated. Results show that the annual value of ET₀ calculated by Hargreaves equation is lower than that calculated by Penman-Monteith equation. ET_0H will be greater than in June, July and August. The calculating value will almost be equal in September, and ET_0H will be lower than ET_0PM in other months. The difference is caused by precipitation and wind speed. Based on the estimating ET₀ by the two equations, the ET₀ calculating equation appliedin arid-area of Northwest China is put forward.