Abstract: Particle flow code (PFC) simulation was performed to investigate meso-response of sand soil under critical flow condition using a coupled liquid-particle model. The effects of hydraulic gradient amplitude and particle stiffness were discussed and the occurrence of sand boil was analyzed from the viewpoint of micro-mechanism. The numerical results indicated that under critical or overcritical gradients, a layer of clustered particles formed near the top wall, and the remainder of the particles formed a suspension and underwent convective-like motion. The thickness of the top layer was proportional to the gradient amplitude. The gradient amplitude based on microanalysis was a little lower than that based on the traditional theory, which reflected the dynamic behaviour of the adopted model. The pore flow was mainly laminar but deviated from Darcy's law under critical or over-critical gradients.