A potential collision hazard between offshore floating wind turbines (OFWT) and ships passing-by needs to be paid further attention as more OFWT fields have been designed and will be built in the coming years. In this paper, a theoretical model is proposed to analyze the dynamic responses of a spar-type offshore floating wind turbine under the scenario of ship impact. Firstly, the collision duration is assumed to be quite short and the impact force is assumed to be large enough to assure all other external forces can be ignored temporarily. The hydrodynamic effects are considered by introducing the added mass matrix during the collision process and then on basis of the rigid body impact principle, and the expressions of energy dissipation and some other kinetic characters are derived. Secondly, the dynamic responses of the OFWT after collision are investigated. In this process, external forces which play significant roles will be taken into account. The hydrodynamic forces are estimated using Morison’s Equation with linear wave theory while the wind thrust force is calculated using BEM methods. The mooring system is considered by the simplification to be a linear elastic spring. Finally, based on the theoretical model derived above, a time-domain analytical program is developed, which can be used to evaluate the dynamic responses for the scenario of spar-type OFWT under ship impact scenario. With the newly proposed program, a series of parametric case studies are conducted and by comparing the results, some characters sensitive to them are further discussed.