To improve the on-site stability of navigational buoys for submarine cable routes under extreme environments and optimize their motion response and mooring tension, this study takes a 2.4 m-diameter all-steel deep-water buoy as the research object, and conducts a comparative analysis of the motion responses of the buoy and mooring tension under single-point mooring (SPM) and three-point mooring (TPM) systems. Under the premise of ensuring consistent total weight and initial pretension of the mooring chains, SPM and TPM systems were designed. A time-domain coupled dynamic response calculation model for the buoy and mooring systems was established, and the buoy’s motion responses and mooring tensions were compared and analyzed under the action of different regular wave heights/periods and the extreme typhoon condition of “Rammasun”. The results show that: the motion responses amplitudes of the buoy and mooring tension are mainly affected by wave height, while influence of wave period is weak; compared with SPM system, TPM system can effectively reduce the amplitude of the buoy’s motion response and the mooring tension. TPM system layout method proposed in this study can provide a reference for the design improvement of the currently commonly used SPM buoys.