The calculation methods of the overall stability of I-section steel beams, under loading both on upper and lower flanges, are not contained in current steel structure code. Energy method was used to derive the critical moment formula of biaxial symmetry I-beams under the loading mentioned above, and the effects of the load scale factor on the critical moment were also summarized. The finite element method was used to verify the moment formula. The results of the finite-element calculation were consistent with the theoretical results when the load scale factor was less than 3. In addition, through computational analysis, the formula of the equivalent critical bending moment coefficient for the I-section steel beams under the same loading was put forward. This formula has a preferable calculation accuracy.