Abstract: Based on the daily climatic data from the nation weather station in 1956-2000,the paper employs the P-M equation and the sensitivity coefficient method to analyze the evolution rule of the potential evapotranspiration(ET₀)and the sensitivity relation of ET₀ to climatic variables in Haihe River basin.The results indicate that the annual ET₀ series appears decreasing trend with four different sub-stages in the recent 45 years.The multi-annual ten-day average ET₀ shows inverse "V" shape, and 61.6% of the whole ET₀ amount occurs in the middle-April to the up-August.In space,the multi-annual ten-day average and the annual ET₀ have an increasing trend from the upstream to the downstream.Compared with the evolution rule of the climatic variables and the sensitivity of ET₀ to them,the evolution rule of the annual ET₀ in the whole basin is similar to that of the multi-annual average wind speed,sunshine time and short wave radiation,and inverses to that of the multi-annual average temperature.The evolution rule of the multi-annual ten-day average ET₀ has same trend to that of the temperature,actual vapor,and sunshine time,as wel as the short wave radiation.However,in all the sensitivity coefficient of ET₀ to these climatic variables,that of ET₀ to short wave radiation is maximum in the whole year and the second water resources districts.The sensitivity of the ET₀ to any other climatic variables is different in time and space,and the values of sensitivity coefficients vary greatly.According to the analysis above,the ET₀cs decreasing trend in the whole basin may be caused by the change of short wave radiation and sunshine time.