Abstract: Combined with the engineering example, structural model was established and the analysis of Modulation Decomposition Response Spectrum and pushover analysis were performed under the action of earthquake for wide flat beam with three different width, . The influence of the ratio of width - to - thickness of wide flat beam on the seismic performance of the structure was analyzed from the aspects of structural modal, deformation, internal force of structure, and plastic hinge distribution. According the analysis, the suggested value of the ratio can be 1.8-3.0. This can provide a useful reference for the design of such structures.