Abstract: A steel-concrete composite beam, whose steel top flange is restrained laterally by the stiff concrete slab, may encounter lateral buckling in distortional mode when in negative bending, which is sharply different from the lateral torsional buckling of bare steel beams. However, the design approach uses the Perry-Roberson formula, which is basically derived from bare steel component tests. To solve this contradiction, inelastic FE buckling analysis is carried out on composite beams in negative bending based on the proposed restrained torsional buckling model. The initial geometric imperfection and the residual stresses are considered. The FE results agree well with the test results. Changing cross-section parameters, 43 groups of composite beams in uniform negative bending are studied. In accordance with EC4, slenderness of the considered beams is defined for, class 1-2 cross-section and class 3-4 cross-section respectively. It is demonstrated that the Chinese Codified steel column design curves can be used in predicting the buckling strength of composite beams with the modified slenderness.