To study the aging mechanism of HTPB(hydroxyl-terminated palybutadiene) propellant, the quantum chemistry method was used to calculate the bond energy of chemical bond of HTPB-TDI(toluene diisocyante) curing system, and transition states of possible four oxidative cross-linking reactions occurring in the aging process of HTPB propellant were investigated. Results show that the bond energy of C—O bond connected with CH₂ group is minimum. The energy is determined as 244.95 kJ·mol^-1. The rupture of the C—O bond in the aging degradation process easily occurs. The activation energies of possible four oxidation linking reactions are all smaller, and less than the energy needed for HTPB-TDI curing molecular degradation, indicating that oxidation linking reaction is the primary cause of HTPB propellant aging. Among them, the activation energy of ternary-epoxy reaction is minimum and its value is 12.59 kJ·mol^-1.