Abstract: Researching the deformation characteristics and mechanisms of ancient landslide deposits in reservoir banks is of significant importance for geological disaster prevention and control in reservoir areas. Taking the Dahua ancient landslide deposit in the Lancang River as an example, based on field investigations and engineering geological analyses, combined with numerical simulations using GeoStudio and FLAC3D and field GNSS measurements, the stability, deformation characteristics, and mechanisms of the Dahua ancient landslide deposit were studied. The results indicate: (1) Under normal operating conditions, the Dahua old landslide deposit is in a stable state, but under abnormal operating conditions, some conditions do not meet the minimum safety factor standards. The stability coefficient of the landslide gradually decreases after water storage, and the deposit undergoes slow creeping deformation. (2) Rainfall is the dominant factor affecting the deformation of the Dahua ancient landslide deposit, and sustained heavy rainfall can cause stepwise displacement events of the deposit. (3) Along the riverward slope of the deposit, slip occurs along the interface between the base and the cover, and the rear slope continuously undergoes creep deformation to achieve a new balance of forces, exhibiting a traction-type deformation characteristic. The research findings can provide references for the prevention and control of ancient landslides in reservoir banks in the Lancang River Basin.