Abstract: At present, the average cost method is used to fix the transaction price of water between the users of water transfer across basins in China, which is easy to cause the problems of repeated inclusion of costs and low efficiency of resource allocation. In order to solve the above problems, based on the theory of marginal cost, combined with the mechanism of water supply cost and water transaction characteristics of water conservancy projects, an inter-provincial water transaction price model for the benefited water transfer users was established, and the method for determining the parameters of the model was proposed. The study calculated water trade price of Beijing City and Hebei Province alone the Phase I of Middle Route of South-to-North Water Diversion Project. The results show that: ① If using existed main line of Beijing section of the South-to-North Water Diversion Project to transfer water from Hebei Province to Beijing City, the transaction price of water is 1.18 yuan/m3. ② If transferring water to Beijing City through the new Gu'an branch line, the trade price of water is 1.40~1.45 yuan/m3. ③ Engineering variable cost is the main component of marginal cost, and the trade price of pressurized diversion scheme is generally higher than that of gravity flow diversion scheme.