Abstract: In view of the high pH value of the water quality of Huilong Reservoir in Yanshan County, Yunnan,the investigation and monitoring were conducted by the ecological environment monitoring station of Yunnan Ecological Environment Department in Wenshan Zhuang and Miao Autonomous Prefecture. The relationship between algae and dissolved oxygen and pH value was analyzed based on the distribution of pollution sources around Huilong reservoir. The factors that may cause the high pH value were examined, and the countermeasures to improve the water quality of the reservoir in Yanshan County were proposed.