The three-dimensional limit equilibrium method cannot search the most dangerous sliding surface when analyzing the slope stability. Taking the Dr2 dangerous rock mass reinforcement project of Suofengying Hydropower Station as the research object, the fruit fly optimization algorithm (FOA) was used to search the sliding surface of the dangerous rock mass. An improved 3D limit equilibrium method was used to calculate the seismic stability of dangerous rock mass. A dangerous sliding surface search method based on the FOA was proposed and the contribution of various reinforcement measures to stability was analyzed. The results showed that the strengthened dangerous rock mass can maintain stability under the earthquake condition, and mudstone grouting contributes the most to the stability, and the top anchor cable contributes the least to the stability. This method helps solve the defect that the traditional limit equilibrium method cannot search the sliding surface, improves the prediction accuracy of slope instability, and can provide some reference for similar projects.