Abstract: A critical safety depth for sunken accident is required in the process of designing the safety shell, which acts as a significant safety guarantee for the offshore nuclear power plant. Since the size of safety shell is large and several kinds of angles will appear when the safety shell is sunk, a method of the critical buckling depth coefficient was developed here for safety shell working under hydrostatic pressure, which can be well applied in the engineering design of safety shell. With the current code computation for safety shell buckling, the process of safety shell working under hydrostatic pressure was carefully simulated and the effects of structural geometric defects on the critical buckling depth coefficient were analyzed. The results show that the critical buckling depth coefficient increases with safety shell’s thickness, yet the increase rate of critical buckling depth coefficient decreases. In addition, the sensitivity of critical buckling depth coefficient on structural geometric defect increases with the decrease of safety shell’s thickness. Without the buckling experiments of safety shell under hydrostatic pressure, this method is a compromised way based on the current buckling design code for pressure vessel under uniform external pressure.