Abstract: The critical dimension of crack instability and propagation of initial elliptical crack of different sizes on the surface of pressure vessel was calculated by FRANC3D. It is found that stress intensity factor of the crack tip before penetrating the wall is less than the fracture toughness K_IC of pressure vessel steel, when the size of initial crack on the pressure vessel surface is less than 3 mm × 20 mm and 5 mm × 10 mm, so the unstable propagation of the cracks does not occur. The cyclic loading numbers of the two initial racks, that are 3mm × 20 mm and 5 mm × 10 mm to penetrate the container wall, are 40688, 45560 and respectively.