The characteristics of the alternating effects of loads in the marine environment make fatigue damage the main failure mode; meanwhile, its randomness and ambiguity bring challenges to the fatigue life calculation and structural design of vulnerable structures. A fatigue life calculation method based on prototype monitoring and bidirectional clustering algorithm based on local density (BCALoD) is proposed for the fatigue damage problem of hinge joints of soft yoke mooring system during service. Firstly, the obtained six degrees of freedom of the hull are classified by the BCALoD algorithm in terms of working conditions, and then the motion data are converted into force time courses using multi-body dynamics as the load spectrum for fatigue life analysis of the hinge joints. The hot spot stresses are calculated after the finite element model of each hinge joint is established by ABAQUS, and the fatigue life is calculated by combining the Miner linear fatigue cumulative damage theory and the rainflow counting method. The fatigue design indexes of the hinge joints based on the measured data are further analyzed and evaluated, and it is pointed out that the fatigue life of the hinge joints on the FPSO soft yoke mooring system is not sufficient to support its completion of service, and it is difficult to maintain and replace each hinge joint uniformly. It can provide a new load treatment method for fatigue life calculation of in-service soft yoke mooring system, which provides a reference for future design of offshore platforms.