Abstract: In order to expand the load cases for calculating the critical moment of simply supported steel I-beams failed by flexural-torsional buckling (FTB), and to reduce the difficulty of calculating the critical moment for combined loads, a formula of critical moment expressed in terms of the critical loads of axial compression members was derived firstly in this paper, as well as a formula of combined moment coefficient in matrix form. Then, with regard to FTB of simply supported steel I-beams, expressions of “3C” coefficient for single concentrated load located at an arbitrary position, and interaction coefficient and combined moment coefficient for two concentrated loads located at two different positions, were derived respectively. The combined moment coefficient for n concentrated loads distributed in equal distance was also derived. Theoretical analysis and finite element software LTBeamN were used to verify the derived results. The results show that, the formulas for calculating the critical moment of single concentrated load located at an arbitrary position, two concentrated loads at two different positions, and n concentrated loads distributed in equal distance, is of high accuracy. For the case of more concentrated loads acting on simply supported steel I-beams, the difficulty of calculating combined moment coefficient is greatly reduced by employing the formula of combined moment coefficient in matrix form.