The ship-borne Stewart platform can compensate for the six-degrees-of-freedom (DOFs) motion generated by the ship, which improves the reliability and safety of offshore operations and increases the executable window period. The heavy and off-center load of the gangway significantly influences the high-precision compensation control of the platform. Besides, the gangway assembled on the platform vibrates easily due to its low natural frequency that requires high dynamic performance of the compensating. To deal with the problem mentioned, the modal space control strategy is introduced to fully consider the inertia characteristics. First, based on Kane's method, the complete dynamic model considering the ship's motion and actuator inertia is established. Then, the modal space proportional and derivative (PD) controller (MSPDC) and the modal space sliding mode controller (MSSMC) are designed based on the modal theory. Finally, simulations are carried out to show the advantages of the proposed model and the advantages of proposed controllers in compensation accuracy and anti-interference ability. Furthermore, the significant compensation rate (SCR) is proposed to evaluate the six-DOFs compensation accuracy. Compared with the PD controller with gravity compensation (PDCGC), the position SCR of MSSMC is increased from 95.37% to 99.28%, and the angle SCR increased from 85.57% to 99.65%.