Aiming at the causes analysis of ammunition spontaneous combustion in the medicine chamber after 60 rounds of continuous projectile, by established a heat transfer model of chrome-plated tube and painted cartridge case during the projectile launch. Select several key factors, such as the thickness of the tube coating, barrel and cartridge case closeness, and the damage of the tube coating, for heat transfer analysis. It is proved that the thickness of the existing paint layer can guarantee the safety of ammunition, which indicates that the main reason for the ammunition spontaneous combustion is the introduction of the gunpowder gas into the longitudinal gas tank.