Abstract In the present study, a series of numerical simulations of the performance changes of a Floating Offshore Wind Turbine (FOWT) with broken mooring line was carried out. For this simulation, an aero-hydro-servo-elastic-mooring coupled dynamic analysis were carried out in the time domain. The OC4 DeepCwind semisubmersible with NREL's 5-MW baseline turbine was selected as a reference platform. One of the three mooring lines was intentionally disconnected from the floating platform at a certain time. The resulting transient/unsteady responses and steady-state responses after that, mooring line tensions, and turbine performance were checked. The accidental disconnection of one of the mooring lines changes the watch circle of the floating platform and the tensions of the remaining mooring lines. In addition, the changes in the platform orientation also cause nacelle yaw error, which is directly related to the power production and structural fatigue life. When horizontal offset becomes large, power-line is likely to be disconnected and its influence was also investigated. To ensure the sustainability of a series of FOWTs associated with farm development, the influence of mooring line failure and resulting changes to the turbine performance should be checked in advance. Otherwise, successive failure of neighboring FOWTs could take place.