Forest ecosystem，the biggest area in the terrestrial ecosystem，is the most important natural ecological system，its service function types are varied，such as soil and water conservation，water conservation，air purification，carbon sequestration，etc. Forest land in Sichuan province accounted for 7.6% of the national forest area. It is one of three big forest areas in China. It is also the largest water conservation area in the upper reaches of the Yangtze River. On the basis of existing literatures review，this paper calculated water conservation capacity of 12.2649 million hectares forests in Sichuan province. In addition，the advantages and disadvantages of the direct market method，substitute market method and hypothetical market method in the aspect of ecosystem service functional evaluation have been compared. The direct market method has been selected to evaluate the value of water conservation of 12.2649 million hectare forests in Sichuan province. The results show that the water conservation capacity is 31.11 billion tons of 12.2649 million hectares forests in Sichuan province，the average water conservation capacity is about 2536.51 t/hm². Its total value is 190.13 billion yuan，the average water conservation value is 15500 yuan per hectare.