Abstract: Through revising the equation of rotation-displacement,the revised factor of the line rigidity of beam with semi-rigid connections is derived.The critical gravity load of structure is derived by the equivalent horizontal load method.The calculation method about the stability of steel frames,which considered the effect of the second order of gravity by controlling the rigidity-to-gravity ratio of structures,is presented.The calculation results show that the characteristic of semi-rigid connections greatly influences the stability of steel frames,and these influences need to be considered in this kind of steel frames.Meanwhile,the influences of the initial rigidity of connections on the rigidity-to-gravity ratio and the deflection of structures are analyzed.