Abstract: TEXAS-Ⅴ is a transient, one-dimensional and three-field mathematical physics analysis code for steam explosion. TEXAS-Ⅴ code was applied to simulate ex-vessel steam explosion of AP1000 under severe accident condition. The results indicate that the molten fuel fragments continuously and transfers heat acutely to the coolant during the mixing phase. AP1000 ex-vessel steam explosion pressure wave intensity gradually decreases during its propagation procedure and it may trigger a stronger pressure wave by causing molten fuel fragmentation after the leading ones. The maximum pressure predicted by TEXAS-Ⅴ is 70 MPa. The ex-vessel steam explosion with larger melt inject diameter, higher initial inject velocity and appropriate mixing time can induce a more violent pressure wave in the explosion phase, which is more dangerous.