The calculation methods of overall stability for HM and HW-shaped steel beams under uniform load on the upper flange and concentrated load on the lower flange are not included in current steel structure code. According to the results obtained with energy theory, with the impact of deformation before buckling on flexural-torsional buckling considered, the critical moment formula of biax-symmetrical HM and HW-beams was derived under the load mentioned above, and the effects of the load scale factor on critical moment were also summarized in this paper. The finite element method was used to verify the moment formula. When the load scale factor was less than 3，the result difference between the finite element method and the theoretical method was within 5%. In addition, the coefficient formula of equivalent critical bending moment for the steel beams under the same loading was put forward. The maximum error of this coefficient formula was less than 8% and the average error was about 3%.