Based on the daily observation meteorological data at habahe、jimunai、fuhai、aletai and fuyun weather station,the paper calculated daily reference crop evapotranspiration by seven methods such as FAO Penman-Monteith,FAO-24 Penman,1948 penman,Makkink,Priestley-Taylor,Hargreaves-Samani,Allen method. FAO Penman-Monteith method was taken as the standard. The paper established a fitting relationship for seven calculation methods and compared the accuracy of calculation method and regional suitability under daily and monthly scales. Results showed that the variation trend of ET0 calculated by seven methods is basic same for five stations on Irtysh river basin, but the values are different. The difference between the results computed by The method of Kimberly penman,Hargreaves-Samani and FAO Penman-Monteith are so small, and they are adaptable for every region. If climatic data are insufficient, Hargreaves- Samani method can provide reliable ET0 estimation. Meanwhile it analyzed the relationship between ET0 computed by seven methods and evaporation of water surface. The evaporation can be used to estimate ET0 by regression formula.