Floating production storage and offloading (FPSO) unit is an offshore oil and gas processing unit that integrates the ship hull system, topside oil and gas processing system, mooring system and export system. The system of FPSO is very complex and easy to cause serious accidents in case of oil and gas leakage. The firefighting system is the important subsystem of FPSO ship system and the important guarantee for personnel and production. The fire situation in the fire compartment has been analyzed to configure the reasonable extinguishing methods aiming at the rationality of the firefighting system design and the equipment selection for the internal turret moored FPSO. A calculation method for the firefighting system of the internal turret FPSO is then proposed based on the relevant rules and regulations. It can improve the safety and reliability of the firefighting system design and ensure the safety of the FPSO production and the crew safety. This firefighting system calculation method is critical to the design of the firefighting system and the equipment selection. It can provide reference for the design of the FPSO firefighting system.