Abstract: On September 18, 2020, a landslide occurred on the left bank of Tianbao Reservoir in Tianbao Village, Sihe Town, Jingning County, Pingliang City, Gansu Province. The landslide is 180 m long, 240 m wide, 48.5×10⁴ m³ in volume, and 191° in main sliding direction, which is the revival of the old loess-mudstone landslide. Landslide destroys the excavated slope and highway subgrade, which seriously threatens the normal construction of Tianbao Reservoir. Through field investigation, geotechnical test and calculation, the geological conditions, basic characteristics and causes of landslide formation in Tianbao Reservoir were discussed, and the stability of landslide was analyzed by simplified Janbu method. The study found that the formation conditions of the landslide are mainly three aspects : the geological foundation of the old landslide, the excavation of the front edge of the slope, and the continuous heavy rainfall. That is, the old landslide is caused by the large-scale excavation of the slope toe and the heavy rainfall. The stability calculation results show that after excavating the slope toe, the stability coefficient is less than 0.98 under the rainstorm condition, which is in the unstable sliding state. After sliding, the stability coefficient of each working condition is less than 1.05, which is in the unstable-unstable state.