Abstract: A damping modification factor (DMF) model for adjusting the vertical acceleration response spectrum of offshore ground motions with a damping ratio of 5% is proposed. Based on 5680 offshore vertical ground motion records from S-net network, the influence of earthquake types on DMF is analyzed by using DMF ratios and Z tests. A DMF model is proposed through considering damping ratios and spectral periods, and compared with the onshore DMF model, by which the vertical response spectra are obtained for other damping ratios. Finally, the residuals and standard deviations of the model are analyzed. The results show that the influence of earthquake types on DMF can be ignored. The influence of damping ratio on DMF can be simulated by a cubic polynomial. Due to the characteristics of offshore ground motions, there are significant differences between offshore and onshore DMF models. The damping ratios can affect the spectral value of one standard deviation of the response spectrum of 5% damping ratio. Residual analysis shows that moment magnitude, fault depth and source distance can be introduced to improve the fitting of DMF model. The DMF model for vertical response spectrum proposed provides a reference for the determination of multi-damping ratios seismic design spectra of offshore engineering.