A comprehensive model test method is proposed to study the stability of rock slope. The comprehensive method involves the strength reduction method based on temperature-dependent similar material and the overloading method by inclined lifting. Corresponding safety evaluation method for slope stability is also established. The left bank high slope in strong unloading area at Baihetan Hydropower Station is taken as a case study. Through two model tests of a typical section and the whole slope in strong unloading area, the deformation features and failure mechanism after reducing the strength of bottom sliding surfaces are obtained. Results reveal that the gently-inclined structural planes are dominant factors of slope stability; the failure mode is block sliding with sliding-tension fracturing. Moreover, results also suggest that concrete replacement plugs in the front area of bottom sliding surface is an effective measure of enhancing the shear capacity of bottom sliding surface, reducing slope deformation and preventing blocks from sliding. The safety factor of reinforced slope is between 1.42 and 1.58. The research results provide an important scientific basis for the slope safety evaluation and the reinforcement treatment of Baihetan Hydropower Station, and also offer reference for similar slope projects.