The normal operation of Yulangpei tailings reservoir is affected by landslide stability. In this paper, taking the main and side slopes near the dam bank of the Yulangpei ditch as an example, water-soil coupling theory is applied to comprehensively evaluate the reliability of the side slopes of the tailings reservoir. Grading and seepage prevention (GSP) measures and the suction of the substrate are considered, as well as the infiltration of different rainfall and reservoir water levels. We numerically simulate the typical three forms of side slopes under the coupling conditions and conduct a reliable and comprehensive evaluation of tailings reservoir side slopes. The study shows that under six reservoir water level changes, the factor of safety (FS) of the bank slope shows a hysteresis effect. According to nine rainfall infiltration conditions and during rainfall, the greater the rainfall intensity, the greater the weakening effect. When rainfall stops, the FS rebounds. After GSP measures, the initial stability of the bank slope under different conditions is improved, but the main slope is more sensitive to changes in rainfall and water levels.