According to the GB150,the slant conical shell subjected to external pressure is designed on the calculation method of conical shell.After researched this method,the equivalent conical shell is subjected less external pressure than the slant conical shell.It results in the thicker design thickness of slant conical shell.Firstly,this paper obtained the critical pressures of cylinder and conical shell by ANSYS-buckling analysis.Secondly,compared results were obtained by ANSYS and theoretical calculation method.It proofed that the finite element method was coincident with theoretical calculation method in external pressure vessels.For slant conical shell of external pressure,finding a conical shell is subjected similar external pressure with the slant conical shell,and namely the conical shell has different cone angle with slant conical shell.The new equivalent conical shell is subjected similar pressure capacity with the slant conical shell,and is improved the original design method.