[Objective] In order to provide scientific support for the prevention and control of water pollution in Yanhe River, the change trend of Yanhe water quality and the main influencing factors were studied.[Methods] An optimal index system was established by Pearson correlation principle to evaluate the water quality of Yanhe River. Based on the system, 283 data series of water quality(2013-2017) were analyzed.[Results] The Nemero index of Yanhe water quality increased and the water quality deteriorated year by year from 2013 to 2017. In the change of Yanhe water quality, the contribution rate of ammonia nitrogen in the evaluation index system increased year by year. It has reached 38.7% in 2017, far exceeding other pollutants. To improve the water quality of the Yanhe River, the total amount of ammonia nitrogen should be mainly controlled.[Conclusion] ① Using an index system including the four factors of ammonia nitrogen, total phosphorus, petroleum and chemical oxygen demand can make the evaluation work more simple and exercisable. ② Controlling the total amount of ammonia nitrogen into the river and improving fresh water supply would reverse the deteriorative trend of Yanhe River.