How to achieve good after-sales service for online retailers in the e-commerce environment is an important issue in China. This paper studies the price strategy of a dual channel retail system consisting of an online retailer and an offline retailer from the perspective of after-sales service cooperation in an e-commerce environment. By comparing the optimal price of both parties and service fee under decentralized decision-making model, this paper takes the optimal price under centralized decision-making model as the benchmark, and maximize the overall profits of both parties and realize profit distribution by applying two-part tariff mechanism, so as to achieve a win-win situation for both parties. Through the service cooperation under two-part tariff, this paper, on one hand, helps online retailers solve the problem of after-sales service, and on the other hand, brings additional service income to offline retailers, which improves profits of both companies. At the same time, unlike the vertical cooperation between manufacturers and retailers in traditional supply chain, this paper innovatively studies the horizontal cooperation between online and offline retailers, and proves that even two competing retailers can achieve win-win situation through cooperation, providing a good theoretical and practical guide for the cooperation and development between competing retailers.