Abstract: The Fushun west open pit mine covers an area of over 12 km², with a maximum depth of over 400 m, and has undergone over a century of mining. While contributing to regional development, it is also accompanied by geological safety issues, such as deformation and instability of the mining slopes. After the stoping of mining, the idea of "water storage into lakes" for the treatment of the mine pit has been proposed. In order to prevent the occurrence of subsequent geological disaster events, this paper takes the potential water-wading soft rock slope as the research object. Through detailed geological survey, geotechnical test and numerical simulation techniques, the mechanism of unstable landslide of soft rock slope during storage period in mining area is investigated. The research shows that: (1) Pit water storage has four effects on soft rock slope, which are soft rock weakening, hydrostatic pressure on the toe of slope, floating weight reduction and seepage force in slope according to influence degree. Soft rock weakening is the main cause of instability of water-wading slope. (2) Under the same intensity of "filling and storage" transformation, the stability of the middle section of the soft rock slope is the lowest. For example, when the pit is filled to −150 m and then water is stored to −50 m, the east and west sections of the soft rock slope are basically stable, while the middle section is unstable and prone to cutting through mudstone and shale to cause landslides. The potential sliding mass is about 133.4×10⁴ m³. It is suggested that during the reconstruction process, special attention should be paid to water-proofing and seepage prevention measures for the rock mass, as well as reinforcing measures at the slope toe in the middle section of the slope. The research can provide reference for the repair and treatment of Fushun West open-pit mine and similar deep open-pit mine.