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Soil–rock mixtures (S–RM) which formed in the quaternary period are a type of extremely inhomogeneous and loose geomaterial with a certain percentage of rock blocks. They are composed of rock blocks with various sizes and high strength, fine grained soil and pores. The meso-failure mechanism and macro-physical and mechanical characteristics of S–RM are largely controlled by its rock block proportion and the granular distribution. As we know, when the rock blocks in the S–RM are larger, it is difficult to take an in-situ sample for an on-site test. In addition, it is difficult to obtain the granular distribution of rock blocks in S–RM by traditional sieving tests. This paper uses a new method called digital image processing (DIP) in which the rock blocks in S–RM samples are separated from the soil matrix, and the proportion and distribution of the rock blocks is obtained quantitatively. The results are used for the sample preparation of the large scale direct shear tests which provide a new method for the test study of S–RM. According to the results of large scale direct shear tests the rock block size proportion controls the deformation and fracture mechanism of the S–RM. The shape of the shear stress vs horizontal displacement curve and the vertical displacement vs horizontal displacement curve of the S–RM samples are different from that of general “soil” and “rock”. With the increment of the rock block proportion the shear band of the S–RM increases. When the rock block proportion lies in the range of 25–70%, the increment of the internal friction angle linearly increases with the increment of the rock block proportion. The cohesion of the S–RM decreases compared with that of the soil. When the rock block proportion is larger than 30%, however, there is only a little decrease in the cohesion with the increment of the rock block proportion.