畸变屈曲负弯矩区是钢 － 混凝土组合箱梁的最重要屈曲模式之一，而钢梁底板的转动及侧向约束刚度是影响其畸变屈曲的关键因素． 文中对钢 － 混凝土组合箱梁负弯矩区钢梁底板的等效侧向及转动约束刚度进行了分析，结果表明钢梁底板侧向及转动约束刚度均与外荷载有耦合关系;基于钢梁底板侧向及转动约束刚度计算公式，利用弹性地基梁法推导了组合箱梁畸变屈曲临界应力计算公式，并进一步获得组合箱梁畸变屈曲临界弯矩;最后通过算例将该方法与 ANSYS 有限元法进行对比． 算例分析表明:负弯矩作用下组合箱梁畸变屈曲临界荷载受构件长度影响较小;负弯矩作用下，转动约束刚度折减系数取 0. 5 时，该方法屈曲弯矩计算结果与 ANSYS 有限元计算结果吻合良好． 该方法利用了更为科学的钢梁底板侧向及转动约束刚度，考虑了负弯矩区底板和腹板的耦合失稳，计算方法物理意义更为明确，同时该计算方法较为简便，为变轴力作用下组合箱梁畸变屈曲临界荷载计算方法提供了理论基础．

Distortional buckling is one of the important buckling patterns of steel-concrete composite box beams un-der negative moment，and the rotational and lateral restraint stiffness of the steel beam bottom plate is one of the key factors influencing the distortional buckling． In this paper，the equivalent lateral and rotational restraint stiff-ness of the steel beam bottom plate in negative moment region is investigated，and a coupling relationship between the restraint stiffness and the external load is revealed． Then，formulas to calculate the critical stress of the flexural-torsional buckling of steel-concrete composite box beams are deduced based on the elastic foundation beam method，and the corresponding critical buckling moments are further obtained． The proposed calculation method is finally compared with the existing methods through examples． Example results show that the distortional buckling moment of the composite box beams under negative moment is only slightly affected by the length; and that，under negative moment，the calculated buckling moments accord well with those obtained by ANSYS finite element method when the reduction factor of rotational restraint stiffness is set as 0． 5． The proposed method makes use of the lateral and rotational restraint stiffness of the steel beam bottom plate in a more scientific manner，and the coupling instability between the steel beam bottom plate and the web plate is considered，so that the method is of great significance in the sense of physics and of great convenience as well． This research provides a theoretical foundation for the calcu-lation of critical stress of distortional buckling of the composite box beams under variable axial force．