In order to explore the spatial and temporal distribution of water quality in the Yellow River basin and the causes of pollution, the monthly monitoring data of 7 basic water quality indicators at 8 provincial boundary stations of the Yellow River from 2015 to 2019 were systematically collected, and the spatial and temporal distribution characteristics and trend changes of water quality of the Yellow River were systematically analyzed by seasonal Kendall trend test method. Based on the surface water quality standard, the status of water quality was evaluated and the main pollution indicators were determined. The linear regression model was used to discuss the causes of pollution. The results show that the spatial distribution characteristics of most water quality indicators are middle reaches > lower reaches > upper reaches. The concentration values of water quality indicators at Tongguan station are significantly higher than those at other stations. TP concentration values at some stations in the middle and lower reaches of the river show a significant upward trend, especially at Gaocun and Lijin stations. From the perspective of spatial variability, the poor water quality of Tongguan station is related to the convergence of several seriously polluted tributaries in this section. From the perspective of seasonal variability, the seasonal distribution characteristics of TP concentration show that the non-flood season is less than the flood season. Under the effect of rainfall erosion in the flood season, agricultural phosphorus carried by tributaries flow into the river with the surface runoff formed by rainfall, resulting in the increase of TP concentration. The seasonal variation of NH3-N concentration in flood season is less than that in non-flood season, which is strongly correlated with air temperature. Higher flow rate and air temperature in flood season contribute to the dilution and degradation of NH3-N.