Abstract: Deep-seated toppling slopes are a common phenomenon in the deep canyon area on the eastern side of the Tibetan Plateau. With the construction and storage operation of an increasing number of large-scale hydropower projects, the instability of deep-seated toppling deformations along the reservoir bank is becoming increasingly apparent. In order to investigate the destabilisation and softening mechanism of deep-seated toppling slopes under the action of reservoir water, this paper takes the phyllite with different degrees of toppling from the Miaowei Hydropower Station′s K73 slopes as the research subject. Through the analysis of macroscopic mechanical properties after different durations of water saturation and microscopic scanning electron microscope structural characteristics, the deterioration law of water saturation softening was obtained. Furthermore, the dynamic assignment of mechanical properties by PFC2D was employed on typical toppled landslides to simulate the evolution process of the toppling slide under the action of reservoir water, and the stability law was obtained. The results indicate that: ① From the standpoint of peak strength, the degree of toppling of rock samples in response to water saturation is not uniform. The magnitude of change in the un-toppled rock samples is relatively minor, whereas the strongest toppled rock samples exhibit the most significant change in peak strength of the water-saturated 60 days. From the perspective of shear strength, the degree of toppling of rock samples in response to water saturation is uniform, showing a downward trend. The decline in the trend of decreasing viscous cohesion is more pronounced in rock samples with a higher degree of toppling compared to those with a lower degree. Additionally, weak toppled rock samples exhibit a much higher decline in this trend compared to un-toppled rock samples. With regard to the reduction degree, the cohesion is greater than the internal friction angle, and the weak toppled rock samples exhibit a markedly higher value than the un-toppled rock samples. ② With regard to microstructure, the principal consequence of saturated water action on rock samples is exfoliation. The exfoliation area exhibits the following pattern: intense toppled rock samples>weak toppled rock samples>un-toppled rock samples. ③ In numerical simulations, the rock parameters solved by the GSI system are employed as a benchmark for calibrating the microscopic parameters. Furthermore, the water-saturated softening law of the toppled rock masses in the preliminary test is utilized as a discounting scheme under the water storage condition. A total of 500 000 simulation steps are conducted to analyze the deformed body of K73, which is divided into four distinct evolution stages: initial creep, bank collapse extension on the outer side of the highway, bank collapse extension on the inner side of the highway, and overall destruction. This study reveals the progressive destruction mode of deep-seated toppling slopes under the softening effect of water storage.