An Unsteady Actuator Line Model (UALM) is developed in this paper and applied to a 5MW floating offshore wind turbine (FOWT). This model is implemented into two-phase fluid CFD solver, naoeFOAMSJTU. The goal of the approach presented here is to investigate the interaction of the aerodynamic loads with the platform motion within acceptable time cost. A semi-submerged floating platform conceptualized in the Offshore Code Comparison Collaboration (OC4) is considered in this paper. Initially the UALM is verified by comparison with the results of a previous study. Next, two kind of fullsystem simulations with different complexity are performed: first, the wind forces are simplified into a constant thrust; second, the fully coupled dynamic analysis with wind and wave excitation is conducted by utilizing UALM. Based on the results, the aerodynamic loads and coupled responses for cases of different complexity are discussed.