In order to choose an appropriate mining method, a synthetic assessment index system was established. A comprehensive evaluation index system model based on the basic theory of the Analytic Hierarchy Process (AHP), the Grey Relation Analysis (GRA) and the weighted TOPSIS was established to optimize the mining methods. During the process, the evaluation indexes in the mining method were comprehensively considered in view of economy, technology and security. The weight matrix of the evaluation indexes was established according to the AHP method, then, according to the weighted TOPSIS improved by the grey relational coefficient, a weighted TOPSIS-GRA comprehensive evaluation model was established, and the synthetic superior degree of the mining methods based on the influence factors was obtained by the TOPSIS methods. Taking the mining method optimization in the transition from the open pit to the underground mining in Shirengou Iron Mine as an example, the synthetic superior degrees of four kinds of optional mining methods are 47.9%,62.5%,63.9% and 59.9%, respectively. According to this analysis, the third mining method is the best. The feasibility and the effectiveness of the evaluation model are verified by the mine's production practice.