To analye the decomposition mechanism of nitroglycerin (NG) during storage in detail, the change in the concentrations of three kinds of gases, including CO, NO, NO₂ released by NG decomposition at 55℃ and 60℃ was monitored by thermal decomposition experiment.The rate constants of four kinds of initial reactions including the hemolytic cleavage reaction of O-NO₂ group, HONO elimination reaction, hydrogen abstraction reaction at α-position and hydrogen abstraction reaction at β-position at 20-60℃ were calculated by the density functional theory and canonical variational transition state theory. Results show that 90 days and 70 days respectively are two important time points at 55℃ and 60℃.With increasing the decomposition time gradually, the concentrations of CO and NO increase, but the concentration of NO₂ increases first and then decreases. At 20-60℃, the rate constant of autocatalytic reaction of hydrogen abstraction at β-position is the fastest, which is 10²-10¹¹ times greater than those of the other three kinds of reactions and the rate of HONO elimination reaction is smallest which is 1/10¹¹-1/10⁵ times of the rate constants of the other three kinds of reactions. The rate of hydrogen abstraction reaction at α-position is about two orders of magnitude more than the rate of the hemolytic cleavage reaction of O—NO₂ group. On basis of the double attribute of NO₂ molecules as the decomposition reaction product and the reactants of catalytic reaction, taking NO₂ gas concentration reaching the highest point in time as benchmark, the process of NG decomposition during storage can be divided into two stages. The hemolytic cleavage of O—NO₂ is the main reaction channels of the first stage, and the rates of autocatalytic reaction of hydrogen abstraction at α=position and β-position increase with the NO₂ concentration increasing. The autocatalytic reaction of hydrogen abstraction at β-position is main reaction channel of the second stage. The autocatalytic reaction of hydrogen abstraction at α-position is secondary reaction channel of the second stage. HONO elimination reaction has minimum effects to NG decomposition throughout the entire process.