The shear strength of soil-rock mixtures (S-RM) is an important parameter affecting the stability of engineering. Therefore, taking the S-RM along Shiyan-Tianshui Expressway (G7011) in Qinba Mountains as the research object, large-scale direct shear tests on S-RM with different rock content and water content were conducted in this paper. The test results show that the shear deformation curves of S-RM samples are basically the same, which can be divided into four stages: elastic deformation, initial yield, strain hardening and shear failure stages. The cohesion of S-RM decreases with the increase of rock content, and the internal friction angle increases linearly with the increase of rock content. The cohesion increases first and then decrease with the increase of water content, while the internal friction angle and shear strength decrease with the increase of rock content. Then, according to the large-scale direct shear test results, the fitting equations of shear strength and shear strength index of S-RM were obtained, and the fitting equations were verified by an engineering example. The results show that the fitting equations were suitable for loess and gravel mixtures with similar properties, and have important guiding significance for the calculation of shear strength of similar engineering materials.