In recent years, due to the influence of reservoir water storage and rainfall, the slope deformation of a large number of reservoirs has intensified, and once the instability is likely to cause harm to the normal operation of the hydropower station. A deformable body on the bank slope of a reservoir, about 6 km away from the dam site of HD Hydropower Station, with a volume of about 3. 53 × 106 m³, was selected as the research object, and the stability and genetic mechanism of the deformable body are deeply studied by comprehensive methods of field geological investigation, laboratory tests, three-dimensional numerical simulation and theoretical analysis. The results show that the bedrock of the metamorphic rock is mainly basalt and tuff, and the overlying layer is generally thin and mainly gravel soil. The continuous development of the metamorphic rock is caused by the coupling of multiple factors. The effective stress of the deformable body decreases, the displacement increases, the shear strain increment of the sliding surface increases and the permeability is better when the rainstorm or reservoir is filled. The 3D numerical simulation results show that the maximum deformation under natural conditions, heavy rain, low water level (1 586 m) and high water level (1 691 m) is 33. 9 cm, 56. 2 cm, 79. 5 cm and 88. 6 cm, respectively. The development process of the deformable body can be divided into two stages, which are the stable stage under natural conditions and the accelerated deformation stage after heavy rain or reservoir filling, indicating that heavy rain and the rise of reservoir water level are the main reasons to promote the continuous deformation of the deformation body. [ABSTRACT FROM AUTHOR]