By the self-made experimental device, the experiment research on the role of relieving differential settlement of subgrade on graded crushed rock in different particle sizes has conducted. The experiment has tested the stress of central, edge and corner of the slab when cavity is produced. The experiment results show that, the graded crushed rock is able to better correspond differential settlement deformation of the subgrade, the liquidity of the bulk particle can fill the cavity spaces. The stress in the slab increases with the addition of the cavity height and the loading. The graded crushed rock with different particles plays different roles on the relief effect of cavity. Generally speaking, the effect of the graded crushed rock in 31.5mm size is better than that of in 19mm size.