To investigate the impact of the cable breakage on the motion response of a ship's mooring system, this study establishes a ship-berth mooring model based on the AQWA hydrodynamic analysis software. First, in the frequency-domain analysis, the six-degree-of-freedom Response Amplitude Operators (RAOs) of the ship are calculated, revealing significant resonance peaks for roll and pitch motions within the range of 0.4~0.6 rad/s. Then, in the time-domain analysis, the C5C6 cable group subjected to the highest load under severe sea conditions (wind scale 7, wave height 2.5 m) are taken as the research object. The motion response of the mooring system is simulated under two scenarios: the cable breakage due to its own damage and the cable breakage caused by exceeding the Minimum Breaking Load (MBL; 1 500 kN). The results show that both breakage cases lead to a significant increase in surge motion (with velocity increments of 2.1 and 6.5 mm/s, respectively), raise tension in the remaining cables by 32%~45%, and increase fender pressure by more than 36%. In particular, a chain reaction of successive failures of adjacent cables is observed within 10 minutes after the initial breakage. The findings can provide references for mooring safety, mooring system design, and emergency response during berthing operations.