Soil-rock mixture (S-RM) exerts a major control on slope stability. Baidian County in southeastern China was selected as a case study to better understand the effect of S-RM on potential slopes. The developed slope deposits in Baidian County mainly consist of different fractions of soil and fragmented rocks with random structure and poor sorting, which is characteristic of a typical S-RM. A series of large in situ horizontal push-shear tests was conducted on selected sites under natural conditions to investigate the mechanical strength characteristics and size effect of the regional S-RM. The results show a larger internal friction angle and smaller cohesion value than reported in previous experimental studies, which directly correspond to block proportion by weight (WBP) of S-RM. The mechanical parameters of the various S-RM sizes exhibit two distinct trends that follow exponential regression functions. The size effect is assumed to be influenced by the non-uniform coefficient (C_u). These results provide important insight on the engineering properties of S-RM and guidance for the design of suitable slope mitigation strategies.