[Objective] The temporal and spatial variation characteristics of water quality in the main stream of the Malian River in the Loess Plateau were analyzed in order to reveal the pollution status and causes of water quality, thereby providing a scientific basis for the construction of ecological civilization and improvement of water quality in the basin. [Methods] Monthly water quality data from 2016 to 2021 for five sections of the mainstream of the Malian River at Qingyang City of Gansu Province in the Loess Plateau were evaluated by using the single factor pollution index method and the comprehensive pollution index method. Trends in water quality evolution were analyzed by using the linear trend method. [Results] ① The overall environmental quality of the Malian River water in Qingyang City from 2016 to 2021 was poor (category Ⅴ), and the comprehensive pollution index was high. The main pollution factors were permanganate index, chemical oxygen demand (COD), five-day biochemical oxygen demand (BOD₅), ammonia nitrogen (NH₃-N), total phosphorus, fluoride and hexavalent chromium. Downstream water quality was better than upstream water quality. ② The seasonal variation characteristics of water quality of the mainstream of the Malian River in Qingyang City were significant. The comprehensive performance was better in summer and autumn than in spring and winter, and the specific performance was best in autumn and worst in winter. [Conclusion] The trend of the water quality of the Malian River in Qingyang City since 2016 has been stable and improved, especially after 2017. Due to the influence of the water quality of the water entering, the river and the geological conditions of the river basin, the water pollution control task for the Malian River still has a long way to go.