In order to investigate the in-service stability of a novel large-capacity floating offshore wind turbine system, hydrodynamic analysis was carried out using the SESAM software package. The influence of key environmental factors (including wave height, wave period, and wind speed) on the platform’s motion responses and mooring line force was investigated. The results show that different environmental factors differ in the intensity of their influence on the dynamic characteristics of the floating platform and mooring line forces, leading to notable differences in platform motion response. It is concluded that the proposed novel floating wind turbine system exhibits favorable hydrodynamic stability, and the findings can provide a theoretical basis and technical reference for the design and in-service performance evaluation of large-capacity floating offshore wind turbine systems.