Aiming at the post benefit evaluation of reservoir reinforcement, the evaluation system of benefit index is built from the economic, social and ecological aspects. The system includes 15 indexes, such as flood control increment benefit, irrigation increment benefit, economic internal rate of return, and water environmental impact. Five evaluation grades and criteria are determined, which are excellent, good, general, bad and disappointing. The modified cloud model(MCM) is brought to calculate comprehensive evaluation. The model combines objective weights calculated by projection pursuit method and the entropy method and initial weights calculated by cloud model(CM), and the index weights are computed by the combination evaluation method. The three characteristic values (Expected value Ex, Entropy En, Hyper Entropy He) of the CM are calculated according to the index grade and standard, and the rank of each index grade can also be calculated by condition cloud generator for comprehensive evaluation. The above method is applied to the evaluation of the reinforcement benefit of the dam in Heiwan Reservoir of Hubei Province, and the validity and applicability of the improved cloud model evaluation method are verified.