The effects of ammonium nitrate on energy of gun propellant were studied. The explosion heat at constant volume, isochoric adiabatic flame temperature, specific energy, covolume and specific volume of propellant gas were calculated by the minimum free energy method. The specific energy and covolume were tested by closed-bomb method. The explosion heat was tested by heat insulation method. The calculated results show that when the contents of ammonium nitrate are 58.49%,50.07% and 43.13%,the explosion heat,the flame temperature and the specific energy of gun propellant are 4743.2 kJ·kg^-1,3075.9 K and 1049.6 kJ·kg^-1, respectively. The testing results show that when the contents of ammonium nitrate are 50.0% and 40.0%, the explosion heat and the specific energy of gun propellant are increased by 23.6% and 5.0% respectively comparing with that of single-base propellant. With the increasing of content of ammonium nitrate, changes of explosion heat and specific energy are consistent with the calculated results.