The deepwater offloading buoy system has relatively small displacement and inertia, so that the mass, damping, and stiffness of the mooring lines have a great impact on the buoy’s dynamic response, thus the accurate coupled analysis of the dynamic response on buoy and its mooring system is particularly important.Applying the boundary element method to calculate hydrodynamic forces of buoy, results in frequency domain can be transformed into the time domain by fast Fourier transform. Based on the total Lagrange formulation and the two-node isoparametric cable element, this paper applies the geometric nonlinear finite element method and Newmark method to calculate tensions in mooring lines. Forth order Runge-Kutta integration scheme is applied to calculate coupled motion equations of buoy and mooring lines in time domain, then the motion response of buoy and tensions in mooring lines can be obtained.