The existing old landslide body is a major safety hazard in the open-pit mining field, and its stability research is of great significance to ensure the safe production of the open-pit mining field. The slope stability of an existing old landslide body in an open-pit mine was studied by using multiple geophysical methods + pattern search method. Firstly, through the seismic imaging method, supplemented by transient electromagnetic method and other geophysical methods, the sliding surface of the existing landslide body was investigated to obtain the position of the existing sliding surface. Then, the pattern search method was used to search the most dangerous sliding surface of the whole slope, and the Bishop method was used to analyze its stability. The results show that the seismic imaging method and the transient electromagnetic method can be used as an effective method to investigate the sliding surface of the existing landslide body after mutual verification. The safety factor of the existing old sliding surface is greater than 1.15, indicating that the sliding surface is relatively stable as a whole. The potential most dangerous sliding surface outside the sliding surface of the existing landslide body has been searched by the pattern search method. A total of eight unsafe sliding surfaces have been found in the P1 slope under the condition of self-weight and earthquake, which are mainly located outside the existing old landslide body, at the top of the slope and below the toe of the existing old landslide body, that is, the position where the slope is steep and varied, and the position with local outburst. A total of three unsafe sliding surfaces have been found in the P1 slope under the self-weight and rainfall conditions, which are mainly located at the lower part of the exit of the existing old sliding surface. P2 slope is basically stable. The research results can provide a reference for slope monitoring and geological disaster prevention.