In order to reveal the aging mechanism of solid propellant grain during the long-term storage, a HTPB (hydroxyl-terminated polybutadiene) propellant rocket motor which was moulded by casting and stored naturally for 19 years was dissected. Samples were taken along the radial position of the propellant grains, and were tested and analyzed by means of SEM(scanning electron microscope), EDX(energy dispersive X-ray spectrometer), infrared absorption spectroscopy and cross-linking density. Results show that the aging degree of propellant is not the same at different positions of grain. The closer the position is to the non-metallic case, the more serious the aging degree of propellant is. Further analysis shows that, in addition to the effects of CC double bond oxidation chain breaking and condensation of —OH on propellant aging, Al³⁺ ions produced by the oxidation of Al powder catalyzes the addition reaction of HTPB′s CC double bond with Cl₂ or water to form CCl double bond and secondary alcohol, and the uneven migration and distribution of aluminum ions will cause the uneven aging of grain. The research results are of great significance to the improvement of propellant aging performance and the research of anti-aging measures.