In order to study the effect of Mg（BH₄）₂ on thermal stability of nitramine explosives， the thermal decomposition properties of Mg（BH₄）₂/RDX， Mg（BH₄）₂/HMX and Mg（BH₄）₂/CL-20 were investigated by differential scanning calorimetry （DSC）. Thermal decomposition products of three mixtures were analyzed by Thermogravimetric analysis-Fourier transform infrared spectroscopy coupling technique （TG-FTIR）. Results show that Mg（BH₄）₂ has different effects on the thermal decomposition and apparent activation energy of three kinds of nitramine explosives， in which the heat release of RDX and CL-20 increases by 14.7% and 32.1% respectively， and but that of HMX decreases by 45.8%. The apparent activation energies of RDX decreases by 15.8 kJ·mol^-1， while that of HMX and CL-20 increases by 19.7 kJ·mol^-1 and 11.5 kJ·mol^-1， respectively. The thermal decomposition products of three kinds of nitramine explosives are the same （mainly NO₂ and N₂O） whether there is Mg（BH₄）₂ or not. Mg（BH₄）₂ has little effects on the thermal decomposition products and the contents of HMX and RDX， but caused the apparent water peak of CL-20. The concentration ratio of NO₂ to N₂O decreased by 89.2%， indicating that Mg（BH₄）₂ promotes the thermal decomposition of RDX and CL-20， and inhibits the thermal decomposition of HMX.