Water resources assessment is an important basis for the implementation of water resources management. In order to meet the stringent water management requirements, this paper proposed some approaches to construct the water quantity evaluation system for Yunnan Water Resources Bulletin based on the theory of water resources assessment and regulations of preparing water resources bulletin. The study examined the specific methods of analyzing and evaluating the precipitation, surface water quantity, groundwater quantity and total water resources, and then developed the water quantity evaluation system for Yunnan Water Resources Bulletin. The system was proved to be valid and feasible in actual application, which can successfully support the compilation of Yunnan Water Resources Bulletin.