A multi-objective model of optimal allocation of water resources was set up based on the maximum overall goals with regard to economic, social, and environmental benefits. The objective weighting factor was obtained with the entropy method, and the comprehensive weight was obtained through a combination of subjective and objective weights, avoiding the weight distribution deviation caused by subjective factors. The firefly algorithm was used to solve the model in a case study. The results show that the algorithm has a higher convergence speed and higher capability in finding the optimal solution, and thus can provide a new method of dealing with complex optimal allocation of water resources.