Abstract To study the reinforcement mechanism of expansive slurry from a mesoscopic perspective, shear simulation tests were conducted on a slurry–fractured rock mass composite using PFC2D (version 5.00.35). The tests analyzed the distribution of cracks, the process of damage evolution, the distribution characteristics of intergranular contact forces, and the displacement of particles. The results indicate that (1) the volume expansion of the expansive slurry compressed the rock mass, causing the slurry particles to penetrate the pores of the rock particles. This process increased the contact area between the slurry and the rock mass, improved the friction, and intensified the degree of interlocking between the slurry and the rock mass, thus improving the bonding between them. (2) Both composite rock masses exhibited similar macroscopic damage patterns consistent with the laboratory tests. During shear tests, both composites experienced four stages of crack development: crack initiation, slow crack development, rapid crack development, and stable crack number. (3) The expansion stress, along with its reaction force and friction force, increased the integrity of the composite rock mass, reducing the differences in particle displacement direction and velocity. This led to improved internal deformation coordination within the composite rock mass, resulting in fewer cracks during shear tests.