Abstract: In this article,we analyze comparatively the two results from Penman-Monteith and Hargreaves of Lhasa reference crop evapotranspiration(ET₀) based on the data from Lhasa weather stations during 1955-2006.The calculated results show the Hargreaves method still has a large deviation in Lhasa in spring and rainy seasons.To further enhance the appropriateness of the Hargreaves methods in Lhasa,we establish and evaluate the method of modifying Hargreaves by introducing the humidity factor,it thus provide a simple method and scientific proof for the accurately determining the water demand of cropand the irrigation schedule in Lhasa region.