Based on the maximum contact force crushing criterion of local stress concentration, by considering the global fragmentation and the local fragmentation, the Apollo filling mode and the direction of maximum contact force were combined to establish a discrete element model to simulate the particle crushing of coarsely grained rockfill, and the large relative density test mechanism of large size rockfill was studied. The results show that there are great differences in the meso-mechanical responses under different crushing modes, and the numerical simulation accuracy can be effectively improved by simulating the local crushing mode of large size rockfill. The higher the rockfill strength is, the greater the effect of gradation deviation on the extreme dry density is. When the ratio of density bucket height to diameter is too small, it is not good for the vibration compaction and particle rearrangement of rockfill. In this case, the measured value of maximum dry density of test gradation may be far lower than the theoretical value, and the relative density may be greater than one if the measured value is used as the discriminant standard for on-site compaction detection of rockfill.