The buckling load of the conical shell is obtained by means of the formulas of many authors. There exists, however, marked discrepancies among these formulas. They are chiefly due to the grade of approximation. The object of this paper is to get a new approximate result and to establish the acceptable method for the conical shell design. The authors develope an acculate expression (12) for the increment of the total potential energy when the conical shell buckles. The buckling load is obtained according to the energy method in the case of the conical shell which has the small cone angle and the comparably short span length. A series of experiments were conducted and the experimental results are in accordance with the present theory.