The end surface of the principal propellant exists regressive burning in large caliber gun. The p-l curve of projectile can be perfected as the regressive burning eliminated. Hereon, different fire retardant amounts epoxy resin composites(ERC) were studied. On the basis of tests of mechanical properties, thermal expansion properties, curing properties of these ERC and open-pore-ratio of coated 25/37 nitroguanidine propellant by ERC, the relationships between the open-pore-ratio and the properties of ERC were analyzed. Results show 99% open-pore-ratio can be achieved when the mass formula of ERC is as follow: 60% resin(E₄₄ epoxy resin:650 polyamide=4:3), 40% mixed fire retardant(quartz powder:TiO₂=1:2), added 10% diluting agent(alcohol:acetone=1:1).