Abstract: Sand-gravel mixture is composed of materials with very different mechanical properties and structures, and the remolded strata composed of the sand-gravel mixture are prone to collapse and other problems. Therefore, the research on the mechanical properties of sand-gravel mixture is an urgent problem to be solved. The shape of gravel is an important attribute parameter in the study of the mechanical properties of sand and gravel. Description of gravel with regular graphics cannot reflect its real mechanical properties. In this paper, the gravel database constructed by digital image processing technology can reflect the true shape of gravel and analyse the specific shape parameters. The particle size distribution of the sand-gravel mixture is wide, and the characteristic particle size cannot be used to describe the overall particle size distribution. In this study, combined with the fractal theory, a double fractal model of the sand-gravel mixture is constructed, and the gradation distribution curve is inversed by the particle size fractal dimension value. Taking into account the discrete characteristics of the sand-gravel mixture, the discrete element software is used to carry out the numerical simulation of the direct shear test and analyse the mesostructure. The results indicate that the sand-gravel mixture generally has two particle size fractal dimensions: the coarse sand particle size fractal dimension value and the gravel particle size fractal dimension value. The closer the fractal dimension of sand and gravel particle size is, the greater the shear strength and internal friction angle. When the values are equal, the sand-gravel mixture has 1-D fractal dimension, at this time the uniformity is the best, the shear strength and internal friction angle are the largest. The axial coefficient is an important parameter to describe the shape of gravel. With the increasing axial coefficient, the gravel shows obvious needle-like properties. In the direct shear test, the anti-rotation ability is enhanced and the amount of surrounding contact is increased, leading to the continuous increase of shear stress and internal friction angle.