Abstract: The seismic response analysis of offshore site is the foundation for the seismic safety evaluation of ocean engineering. Seismic wave input is a key part of site seismic response analysis. However, the slope of the offshore site inclines to the ocean, which interacts with the seawater directly, leading to natural difficulties in solving the free wave field and thus provides a challenge to the existing seismic wave input methods based on the free wave fields. In this study, we adopted and improved the seismic wave input method based on the substructure of artificial boundaries, and constructed a hybrid input wave field with the free wave field on two lateral boundaries and the incident wave field on the bottom boundary of the calculation model. On that basis, an offshore site seismic wave input method for vertically incident P and SV waves is proposed. The proposed method avoids calculating the free wave field of irregular offshore sites, therefore it can simplify the seismic response analysis of offshore engineering considering characteristics of offshore site and the soil-structure interaction. Meanwhile, the fluid-structure coupling algorithm based on the acoustic fluid element is used to simulate the dynamic interaction between the offshore site and the seawater, and the fluid dynamic artificial boundary and the uniform viscoelastic artificial boundary element are used to simulate the radiation damping of the infinite fluid and solid medium. On that basis, the dynamic interaction model of offshore site is established. Combine this model with the proposed seismic wave input method based on the hybrid wave field, thereby an overall time-domain analysis method for the seismic response of offshore sites is established. Numerical examples show that the analysis method and numerical model proposed in this study can accurately realize the input of seismic waves in offshore sites, and have good applicability for seismic response analysis of offshore engineering.