Development and implementation of security measures for storage of ammonium nitrate are lack of appropriate reference data, because of few studies for thermal characteristics of ammonium nitrate after long-term storage. In order to address this situation, the thermal decomposing parameters of ammonium nitrate, which have been stored for 10 years, 20 years, and new produced, were measured by using adiabatic accelerating rate calorimeter. The test data was amended by means of thermal inertia factor Φ. It was showed that when compared with the new produced ammonium nitrate, the initial decomposition temperature of ammonium nitrate stored for 10 years and 20 years was reduced by 42.26℃, 58.02℃ respectively; the maximum decomposition temperature was improved by 20.28℃ and 179.55℃; the temperature rise was increased by 62.54℃ and 255.57℃; the maximum rate of temperature rise was improved by 31.1 times and 99.3 times. Through the mechanism analysis of the long-term storage of ammonium nitrate, due to the dual catalytic nitric acid and water, the ammonium nitrate contained nitric acid and water, which led the thermal hazard of ammonium nitrate after long stored in room temperature deteriorated. The tests conclusions can be the beneficial reference for the store safety of ammonium nitrate.