Abstract: A simplified analysis method of steel-concrete composite frame structures under rare earthquake action is established based on the ultimate balance theory. The failure mechanism of composite frames is determined by theoretical analysis and experimental phenomena. Based on the ultimate balance theory and according to the distribution law of the deformation and internal force when the structure reaches the ultimate strength under earthquake action, the seismic strength of the composite frames is calculated when the top horizontal displacement of the frame is equal to H/50, H/40 and the lateral displacement corresponding to the limit load. The calculation results agree well with the low cyclic loading test results of 19 composite frames at home and abroad. It shows that the method can effectively predict the seismic strength of composite frame structures under rare earthquakes. A simplified method is proposed to control the deformation capacity and seismic strength of the composite frame at the same time to make the structure meet the seismic requirement under rare earthquakes.