The influence of tightness on the storage stability of GATo-3 propellant was studied by ageing test,gas phase chromatography (GC) and microcalorimeter. The influence rules of tightness on the storage stability of GATo propellant at 55 ℃,65 ℃ and 75 ℃ are different,which is caused by sublimation of resorcinol (Res), catalysis of H₊ and sealing pressure. Sealing pressure accelerates the thermal decomposition of GATo propellant, which is proved by microcalorimeter.