The Wangjiadun landslide in Qin'an County of Tianshui City passes through the Qin'an tunnel and is a giant ancient landslide group along the Baoji-Lanzhou PDL. This study selects the Wangjiadun landslide to assess the static and dynamic anti-sliding stability problems in the project. Internal factors such as geological structure and site engineering condition, which affect the stability of the Wangjiadun ancient landslide, are evaluated through laboratory tests and field investigation. The dynamic response of the Wangjiadun ancient landslide under seismic load is analyzed by dynamic finite element method. The dynamic anti-sliding stability is studied using the combination of dynamic finite element method and strength reduction method. Displacement mutation is employed to determine the dynamic instability and dynamic safety factor of the slope. Results show that the horizontal thrust of the earthquake has a great influence on the stability of the Wangjiadun ancient landslide. The effect is represented by the overall slip of the upper slope and the collapse of the loess deposit in the entrance section of the tunnel. In the natural state, the slope is in the stable state. In the event of strong earthquakes in the region in the future, the slope will be unstable. The overall sliding surface of the loess slope is most likely to occur in the third slope body, along with the sliding surface with the maximum plastic strain. The safety factor F_S is 0.92.