Based on a thirdparty platform for evaluating supplier sustainability, transnational companies can effectively monitor how much efforts and improvements their global suppliers have made in sustainability. However, methods on the basis of the platform for evaluating and calculating the weights of indexes and values suffered from a lack of accuracy. A stochastic analytical hierarchy process (SAHP) method and the triangular fuzzy number were adopted to optimize the evaluation model. Besides, a practical case about the sustainability assessment of suppliers was analyzed by the original method and the improved method separately. The comparative results show that the proposed model successfully differentiates the sustainable performance of suppliers and obtains a distinct rank.