Recently, we usually use the numerical method to calculate the buckling strength of plate and shell structures. But at early design stage, we even now use the theoretical or some simplified formula to check the buckling strength. For most of typical plate and shell structures, the precise formulas of buckling strength have been developed. For the general buckling strength of ring-stiffened cylindrical shell under external pressure in elastic range, the several formulas have been also proposed, but the accuracy of those are hardly adequate. In those formulas, the effective breadth of ring-stiffeners is introduced, but the exact value has not been presented. In this paper, the calculation procedure of effective breadth is developed based on mechanical consideration of pre-buckling deformation, and the modified simplified formula of general buckling strength of ring-stiffened cylindrical shells under external pressure in elastic range is proposed. Finely, the accuracy of proposed formula is confirmed by comparison with numerical results.