In order to understand the resistance of grid frame structure under horizontal loads，based on D-value method，assuming that axial force is linearly distributed on structural cross section and considering the influence of bending and shear deformation，this study deduced the equivalent lateral resisting stiffness formula under inverted triangular load，uniformly distributed load and vertex concentrated load. The results were compared with that of finite element method and experiment．The results show that lateral displacement of this structure is given priority to shear deformation，and with the increase of structure height，the influence of bending displacement becomes larger，so that the effect of bending displacement on the total lateral displacement should be considered. The influence of axial deformation of middle column on structural lateral resisting stiffness and displacement is smaller．Compared with the calculated lateral stiffness by finite element method or test results，the equivalent lateral stiffness formula in this paper has reliable calculation accuracy，and can be used to estimate the capability of resisting lateral deformation．