Abstract: To evaluate the damage degree of Composite Frame with Reinforced Concrete Column and Steel Beam after the earthquake, the seismic damage model of RCS composite frame structure was established by modifying the relevant parameters of the Park-Ang double parameter seismic damage model. By analyzing the influence of the slab on the key parameters of the model, the formula to calculate the ultimate deformation capacity of the composite beam members under the influence of the slab space combination effect was given. The formula was verified. Based on the test results, the parameter β under the influence of cyclic loading of the composite beam member was obtained by inverting the component at a damage index equal to 1 at the point of failure. The range of damage index corresponding to the degree of damage of the structure was given, and a seismic damage assessment method for this structure was proposed. The seismic damage assessment of a 4-layer RCS composite frame structure under the action of El-Centro wave shows that the seismic damage model proposed in this paper can better reflect the damage evolution process and degree of RCS composite frame structure. The research results provide a theoretical basis for seismic damage assessment and performance-based seismic design of RCS composite frame structures.