A complex mode superposition method based on real modes of the undamped system is proposed to improve the calculation efficiency of forced vibration of the non-proportional damping system. The complex eigenvalues and modes are represented by the linear combination of real modes through the modal perturbation method. So the forced vibration solution can be expressed by the real modes. Then，the seismic response is analyzed by a three-story frame structure with additional damping which is a strong non-proportional damping system. The result shows that eigenvalue errors are less than 2% when the number of additional real modes exceeds 8. The numbers of complex modes calculated from the cumulative mode participating mass and modal contribution factor error of displacement are not enough，and the modal contribution factor error of acceleration is suggested to be a better modal truncation index. Compared with the state-space method，the proposed method is more efficient，the peak errors of the displacement response are less than 10%，and the cumulative errors are less than 15%，by a few low modes determined by modal contribution factor error of acceleration. Finally，the applicability of the proposed method is verified by seismic response analysis of an office building with a steel frame. Overall，the proposed method can balance calculation efficiency and accuracy for highly non-proportional damping systems，while the real mode superposition method with forced decoupling shows large errors.