Based on the Budyko hypothesis， this paper improved the water-energy balance equation integrating soil water storagevariable factors， and verified the adaptability of Budyko hypothesis in the Songhuajiang River basin， and analyzed the spatio-temporalvariability of actual evapotranspiration and its driving mechanism. The main results are as follows： （1） taking 1995-2006 asthe model parameter calibration period and 2007-2012 as the model verification period， using P-ΔS to represent the water supplyconditions for land surface evapotranspiration， which indicate that soil water storage variable factors have improved the calculationaccuracy of actual evapotranspiration. （2） Under the significance level of α=0.1， there is no significant trend in the ETa of eachcatchment from 1995 to 2012； the spatial distribution of ETa gradually increases from the west to the east and has obvious zonality.（3） The spatial variation of the contribution of precipitation to ETa is consistent with the spatial variation of total contribution ofthe potential evapotranspiration， precipitation and soil water storage variables to ETa， which indicate that the water condition of theSonghua River basin is the dominant factor affecting the actual evapotranspiration.