Abstract: The mechanism of ejecting gas-shades of COIL was given. The effect of ejecting gas-shades on pressures and output power of COIL was studied. The result shows that by adopting the action modes of ejecting gas-shades in the optical resonator, the loss of stagnation pressure is remarkably reduced. The working load of the pressure recover system at COIL exit will be lightened. On high back-pressure condition, static pressure in the resonator is remarkably reduced then the stability of static pressure in the resonator is improved.