<p>Benefit evaluation of debris flow prevention and control projects was one of the essential contents of debris flow prevention and mitigation work. In order to scientifically and quantitatively evaluate the comprehensive benefit of debris flow prevention and control projects, this study identified nine factors as evaluation indicators from economic, social, and ecological aspects. The projection pursuit (PP) model based on the improved particle swarm optimization (IPSO) algorithm was used to construct a mathematical model to evaluate the benefit of debris flow prevention and control projects. The interpolation method was applied to divide the benefit grades. The debris flow prevention and control projects in Qipan, Taoguan, Chutou, Anjia, and Mozi gullies in Wenchuan County were chosen as typical cases for empirical analysis. The case study revealed that, among the criteria layer indicators, investment per unit of the protected area, investment per unit of the protected population, the amount of water and soil conservation, and reduction rate of accumulation fan had the most significant weights. The social and ecological benefits were found to be the more important in the target layer. The comprehensive benefit of Qipan, Taoguan, Chutou, Anjia, and Mozi gullies was found to be 4.44, 4.83, 1.95, 3, and 2, respectively. The benefit ranking of the five gullies was consistent with their effectiveness in disaster prevention ranking in the flood season of 2019. Therefore, it could prove that the newly-built benefit evaluation model was practical and feasible, and the evaluation results of the sample could be reasonably interpreted, which verified the effectiveness of the methods.</p>