Conglomerate formations are important unconventional oil and gas reservoirs. However, the conglomerate reservoirs are heterogeneous because of the gravel particles. Therefore, theory based on homogeneous rock does not apply, and the law and mechanism of fracture propagation are urgently needed in conglomerate reservoirs. To study the influence of gravel content and cementation on rock deformation, crack propagation and morphology, we established a discrete element model simulating uniaxial compression based on the PFC2D software. The results indicate: (1) The stress-strain curve exhibits stress fluctuation during loading. The underlying reasons are gravel rotation and the interaction between the crack and gravel. (2) Networks of shear cracks around the gravel will be easily formed in the weakly cemented conglomerate. While in the strongly cemented conglomerate, the crack will penetrate the gravel to form a single fracture. (3) Cracking tends to occur at the cementing surface compared with gravel and matrix. In addition, the gravel will change the cracking path. Therefore, the mesostructure of the conglomerate controls the fracture network complexity. These results can theoretically guide hydraulic fracturing in conglomerate reservoirs.