To obtain the effect of water wash on improving civil aero-engine performance, realize the transition from a hard time water wash to on condition water wash, and provide support for assessing the effectiveness of energy saving for an airline, an effect evaluation method of aero-engine water wash is proposed. The method chooses the extending time on wing and the saving fuel quantity as main evaluation indicators of evaluating the effect of aero-engine water wash, and calculation methods of evaluation indicators are put forward. The effect evaluation model based on cost of aero-engine water wash is established, and water wash effect can be evaluated based on the model. The proposed method is validated by the actual data from an aero-engine after water wash. Results show that the proposed method can achieve a quantitative evaluation of the effect of aero-engine water wash. Finally, an aero-engine water wash management system is developed based on the proposed method. This study can provide support for the implementation of on condition water wash, and also provide support for assessing the effectiveness of energy saving for an airline.