Abstract: The coupling effect of multi-stage impoundment cycles and extreme rainfall in large reservoirs often induces complex deformation evolution and sudden instability of reservoir bank landslides. The associated dynamic response mechanisms and instability mode transitions have become core scientific issues in the prevention and control of geological disasters in reservoir areas. Taking the Shenjiagou landslide in the Baihetan reservoir area as the study object, this paper, based on detailed field geological surveys and long-term InSAR deformation monitoring data, designed and conducted large-scale physical model tests encompassing four typical working conditions. The evolutionary characteristics of pore water pressure, earth pressure, and macroscopic deformation within the landslide under the coupled influence of periodic reservoir water level fluctuations and extreme rainfall were systematically monitored. The results show that: ①Under the combined action of reservoir water and rainfall, the Shenjiagou landslide exhibits a stepwise retrogressive progressive failure mode characterized by front-edge traction, middle-section propagation, and rear-edge coalescence. ②The influences of reservoir water and rainfall on the internal stress field of the landslide are significantly different: reservoir water level fluctuations primarily control pore water pressure changes at the front edge and induce local deformation there, whereas rainfall infiltration triggers overall deformation and failure by altering the seepage field characteristics within the landslide mass. ③The superposition of extreme rainfall significantly changes the critical instability mode of the landslide: under normal reservoir water action, the deformation is dominated by buoyancy forces and failure mainly occurs during the water level rising stage; however, after superimposing extreme rainfall, the combined effect of a sudden increase in pore water pressure and hydrodynamic seepage force greatly reduces the overall stability of the slope, causing the dominant deformation and failure stage to shift from the rising stage to the falling stage of water level. ④Through synergistic analysis of multi-physical monitoring sequences under instability conditions (Condition I and Condition IV), precursory information of landslide failure was extracted. The rate of earth pressure change exhibits a sudden, sharply increasing negative mutation near failure, indicating a significant enhancement of slope unloading. Based on the coupled criterion of displacement rate and dynamic earth pressure changes, the landslide can be accurately determined to have entered the critical accelerated creep stage. This study reveals the deformation characteristics and progressive failure mechanism of landslides subjected to multi-stage hydrodynamic action, and achieves effective extraction of instability precursory information, providing a theoretical basis and technical support for precise early warning and disaster prevention and mitigation of large-scale soil landslides in giant hydropower station reservoir areas.