The mixture mixed by crushed sandstone and mudstone particles are gradually used as a kind of fillers in geotechnical engineering. However, the engineering practice goes ahead of the theory, the engineering features of this particle mixture have not been well revealed. Based on series of direct shear tests in laboratory, shear strength of the sandstone–mudstone particle mixture and its influencing factors are investigated in this paper. The curves of nominal shear stress against relative lateral displacement for test materials are obtained. Particle size distribution, mudstone particle content of experimental materials, initial dry bulk density and water content of test specimens are listed as four important influencing factors and they are carefully analyzed with single factor analysis method. The rule of the four important factors influencing on the engineering features of this particle mixture are derived, respectively. The experimental results can be used as a reference for this kind of filling project. The conclusion of this paper is beneficial to the popularization and application of the mixture with crushed sandstone and mudstone particles.