Abstract: When the claddings of the PWR fuels breach, the fission gas accumulated in the gap of pellets and claddings releases into the coolant. Naturally, the process of fission gas release is a process of two-phase flow, while the microscopic mechanics is still undiscovered. To reveal the law of the interaction between the coolant and the gas during the process of fission gas release, a CFD method is built to simulate the transient process of fission gas release, in which VOF and k-ε model is used. The result shows that the coolant will flow into pellet-cladding gap, then it will be evaporated, which will lead to the increasing of the pressure in the gap, and the fission gas will release into the subchannel; the processing that the fission gas releases from the pellet-cladding gap can be divided into 2 steps. In step 1, the differential pressure between the pellet-cladding gap and the subchannel is relatively larger, which makes the gas jet into the subchannel, and the duration of this step is short, and the ratio of the release of the fission gas is higher, meanwhile, it changes violently.  In step 2, the difference of the pressure between the pellet-cladding gap and the subchannel is relatively smaller and stable, and the fission gas enters the subchannel by the convective mass transfer through the vortex in the breach. The duration of this step is short, and the ratio of the release of the fission gas is lower and stable.