In order to reduce the underwater radiated noise of WCS (water-filled cylindrical shell) excited by an interior point acoustic source, a gas-bag was coated on the outer surface to form the gas-bag coated cylindrical shell. In order to give instructions on the design of GCWCS (gas-bag coated water-filled cylindrical shell), the bare and the gas-bag coated WCS were respectively simplified into the single-leaf and the double-leaf partitions. By comparing the wave impedances of the partition, gas and water, the influences of the sound velocity and thickness of gas on the low frequency radiation of double-leaf partition with plane incidence sound wave were analyzed. Analysis results show that gases with smaller sound velocity and proper thickness have better radiation noise reduction effect. The acoustic infinite element method was employed to calculate the underwater acoustic radiation of GCWCS, whose results are in good accordance with those of the partition analysis. The underwater radiated sound power and the far-field radiation sound pressure of the optimized CO₂-bag coated WCS are much lower than the bare one.