Making clearness of the hydrological cycle process of mountainous watershed is very important for the sustainable development and ecological environment protection of the inland river basin in Northwest China.Using numerical methods,we create several statistical models to calculate the monthly evaporation capacity of Heihe mountainous watershed.The results show that the monthly average air temperature,the monthly evaporation capacity and monthly precipitation has a highly nonlinear relationship between each other.Precipitation and air temperature both are the comprehensive factors that effect the variation of the evaporation.