In order to improve the fire-fighting efficiency of fire-fighting aircraft, the discharge characteristics of water extinguishing agent by establishing a working medium discharge model of gravity spraying system is simulated, the influence of the type of working medium and the size of water tank on the discharge process is analyzed. The results showed that the discharge rate in 6 s is 9% slower than that of water when the polymer gel extinguishing agent is used instead of water as the working medium, which meets the emission requirements in actual application scenarios. However, the commonly used shrinkage criteria are no longer applicable to the discharge process of water tanks of different sizes, so the shrinkage criteria applicable to the gravity spraying system are established, and the shrinkage test platform is built to verify the emission model. The results show that the most suitable shrinkage criteria number for the gravity spraying system is Froude number.