<p>Soil–rock mixture (S-RM) is a natural geomaterial characterized by multiple grain size grades and pronounced trans-scale hierarchical features. To investigate the multiscale coupling shear strength of S-RM, the material is decomposed into a soil matrix and rock blocks based on the deformation characteristics of mineral particles at different scales. A multiscale soil–rock cell element model is proposed. Moreover, three groups of in situ large-scale direct shear tests were conducted on S-RM samples with varying rock block contents to calibrate the microscale parameters of the model. The results show that rock blocks significantly affect the mechanical behavior of S-RM. The trans-scale, coordinated deformation between the soil matrix and rock blocks induces strain gradients at the mesoscale and microcrack coordination at the microscale, particularly at the matrix–block interfaces. This interaction leads to increased energy release at the macroscale compared with that of the pure soil matrix. This phenomenon constitutes the multiscale physical mechanism underlying the shear strength enhancement due to rock blocks. The proposed model accurately predicts the shear strength of S-RM, consistent with experimental observations.</p>