On the basis of analyzing the impact of ecological water supplement on water environment, improved evaluation system is proposed from the angle of hydrology, hydraulics and water quality. Specific evaluation indexes include the quantity of ecological water supplement, efficiency of water supplement on important joint, change rate of water exchange cycle and improvement degree of water quality in the district. Taking ecological water supplement experiment in Changshu as the study case, improvement of water environment caused by ecological water supplement was analyzed and evaluated. The results show that: This index system can be a general method to evaluate the effect of ecological water supplement on water environment, and can also be used to select the optimal simulation scheme.