Abstract: Not like the traditional fluid or solid, granular media is a combination of large amounts of discrete particles, which may lead to the coexistence of di erent flow regimes and their complex transitions between each other in granular media during its movement. Elastic instability and irreversible flow rule are the keys to understand the mechanical behaviours of granular solid and granular fluid, and especially the phase transition from granular solid to granular fluid. In this paper, we deduced the instable critical condition for instant elastic energy based on the two-granular-temperature thermodynamics (TGT) for granular media, and developed a related irreversible flow rule, with a MPM numerical model we built and used it to simulate a sandpile collapsing, which is a typical transition process between granular solid and granular fluid. Physical experiment and DEM simulation were carried out for comparison, and both numerical results can march well with those from the experiment. We also found out some disadvantages for TGT model and MPM. In this work, the sand dam developed from static compression to irreversible flow after failure, then finally deposited again, and the TGT model and MPM shows the capacity of describing the solid- and fluid-like behaviour, and the phase transition of granular media well during the sandpile collapse.