The P-Δ effect is one of crucial reasons causing the structure to collapse under the excitation of strong earthquakes. Under the interaction of the geometric nonlinearity and the material nonlinearity, the backbone curve of structure behaves a negative post-yield slope. In order to calculate the structural response function rapidly, the proportion rule between the seismic intensity and the normalized yield strength for the similar inelastic systems under the same ground motion is revealed. Based on this rule, the relationship curve between the normalized yield strength η and the ductility factor μ of the equivalent single-degree-of-freedom system is established, and the simplified procedure to calculate the seismic intensity of each point in the pushover curve is derived, which generates the structural response curve. A numerical example of six-storey steel building is conducted, calculating using the proposed procedure and the exact incremental dynamic analysis method respectively for an ensemble of 20 ground motions to obtain the 16%, 50% and 84% fractile response curves of the roof drift ratio, which demonstrates the simplified method agrees with the IDA from elastic, inelastic to collapse.