There are significant differences in the structural characteristics of conglomerate reservoirs, among which the presence of gravel not only leads to strong heterogeneity of conglomerate, but also has a significant impact on the mechanical properties and fracture development of conglomerate. At present, the influence of the composition of different particle sizes of gravel on the mechanical properties and crack development of conglomerate is still unclear. This article cites the concept of inter-mediate gradation in soil mechanics to characterize the composition of different particle sizes of gravel, and quantifies the grading characteristics using the curvature coefficient Cc and non-uniformity coefficient Cu as grading parameters. A numerical model of conglomerate based on particle flow PFC2D simulation software is established. The influence of gravel gradation parameters on the mechanical properties and crack propagation of conglomerate is investigated by analyzing the stress-strain curve, compressive strength, elastic modulus, and crack development status. The findings reveal that the compressive strength of conglomerates is primarily influenced by the non-uniformity coefficient Cu and the average particle size of gravel, exhibiting a negative correlation with Cu. Similarly, the elastic modulus of conglomerates correlates with the average particle size of gravel. The number of cracks is affected by both the curvature coefficient Cc and the average particle size of gravel, demonstrating a negative correlation with Cc. Conversely, the number of cracks positively correlates with the complexity of the fractures. These research findings offer novel insights into exploring the mechanical properties and fracture propagation patterns of conglomerate reservoirs.