The hydrodynamic response analysis and time-domain coupling analysis are carried out for a submerged external transport buoy conceptually designed in the actual development project of the marginal oilfield in the South China Sea. The effects of different damping ratios and geometrical shapes of the buoy on the hydrodynamic performance and time-domain motion response of the submerged external transport buoy and the tension of the mooring lines are studied. The results show that: the larger the damping is and the longer the geometry of the submerged buoy is, the smaller the time domain motion response of the buoy and the tension of the mooring lines are; the geometrical shape is of different influence on the frequency domain and time domain motion response of the buoy.