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．