Expansive soils are widely distributed in the Danjiangkou Reservoir region (DRR), China. In recent years, with a rise in the reservoir water level (RWL), reservoir-related expansive soil landslides have gradually become a serious issue in this area. However, few studies on reservoir-related expansive landslides have been reported. Taking the Jingang landslide as a case study, a detailed analysis was performed to interpret the movement characteristics of the landslide under the influence of rainfall and the RWL, based on the comprehensive in-situ monitoring data and laboratory soil tests. The Jingang landslide is a large-scale expansive soil landslide triggered by reservoir operation. Available data indicate that the displacement of the Jingang landslide displays step-like trends and mainly occurs during high RWL or filling operation periods. Both seasonal rainfall and RWL variation are important factors that cause the ground movement of a landslide. The onset of rapid ground movement usually corresponded to an RWL of 161–163 m a.s.l.. The motion pattern is primarily retrogressive, and the displacement velocity increases towards the toe, which could reach 41 mm/month. The free swelling rates of the soils in the Jingang landslide are approximately 40–60%. The stability calculation indicates that the factor of safety of the Jingang landslide decreases from 1.049 to 1.038 due to the swell pressure of expansive soil, and this has a significant adverse effect on the landslides.