Inorder to quantively analyze the evaporation rules of plain reservoir in the Aksu river basin, the paper used the observational data in the Aksu water blance experiment station from 1981 to 2010 to analyze the evaporation change of the three plain reservoirs within the year and inter year,and emphatically analyzed the relationships between water surface evaporation and water surface area of reservoir, evaporation volume and temperature, and quantitatively analyzed the effect of surface evaporation on reservoir evaporation. The results indicate that the inter annual variation of evaporation is larger, evaporation mainly concentrates in April to October.The amount of evaporation in three reservoirs ccountes for 88.14%, 87.02% and 87.6% of that of total year respectively. The interannual variation of evaporation is smaller.The evaporations of the three plain reservoirs had increased by 1.5, 0.56 and 0.18 percent from 1981 to 2010 an average annual.