The invention relates to a floating roof tank floating disc type compressed gas foam fire extinguishing system and a fire extinguishing method. The floating roof tank floating disc type compressed gasfoam fire extinguishing system comprises a compressed gas foam generating device, a fire control valve, a fire control main pipe, a flexible fire control pipe, a fire control branch pipe and a plurality of compressed gas foam discharging units. When a fire occurs, the compressed gas foam generating device is started, high-stability compressed gas foam is generated according to the preset flow andconveyed to the compressed gas foam discharging units through the fire control main pipe, the flexible fire control pipe and the fire control branch pipe, the foam is evenly and precisely applied toa sealing ring area between a metal rain baffle and a primary sealing device through foam guiding pipes of the compressed gas foam discharging units, and the fired portion is directly covered. The floating roof tank floating disc type compressed gas foam fire-fighting system has the effects that ultra-fast fire extinguishing is achieved, the fire extinguishing speed is increased by 10-30 times, the fire loss is reduced to the maximum limit, the foam stability is good, the reliability is high, the reignition resistance is high, high-risk personnel do not need to climb to extinguish the fire, and the effectiveness and the reliability of the system in a fire explosion environment are greatly improved.