This paper presents the dynamics of a thruster assisted mooring system. Mooring is used to maintain the position of a floating platform against quasi-constant external forces induced by current and wind, and thrusters control slow drift oscillation. The LQI (linear quadratic with integral) control system was adopted to prevent this oscillation. Control of motions in the wave frequency region was ignored because the amplitude of those motions is smaller than those of the slow drift oscillation and enormous power by a thruster is needed to control them. Hydrodynamic coefficients of the floating platform were obtained by model experiments and calculation using a three-dimensional Singularity Distribution Method. Simulations of motions in irregular waves, current and wind were carried out and the performance of LQI control system was discussed.