Abstract: In order to study the rationalization of the total potential energy equation of steel members with flexural-torsional buckling, the deriving procedure of stress method and strain method is analyzed in this paper. It is found that the expressions of strains and displacement mode are the two primary factors that affect the results, then two criteria on the rationalization of the two primary factors are established, and the correctness of the two criteria is verified. By using the two criteria, two typical displacement modes and three typical total potential energy equations are judged. The conclusion showed that Vlasov’s displacement mode is more reasonable than Trahair’s, and Lü’s total potential energy equation is more rational than Bleich’s and Trahair’s. Finally, a total potential energy equation which should be adopted for deriving the design formula of critical moment Mcr is proposed.