The thermal decompose characteristics and combustion performance of high-energy TEGN propellant that containing hexogeon(RDX) and thermoplastic polyurethane elastomer(TPUE) was analyzed by differential scanning calorimetry(DSC) and closed-bomb. The combustion mechanism of high-energy TEGN propellant was analyzed and the interrupted burning surfaces was observed by 3D microscope. The results show that the thermal decomposition process of high-energy TEGN propellant mainly contains the thermal decompose of basic body which consists of nitrocellulose(NC), nitroglycerine(NG), nitrotriglycol(TEGN) and the thermal decompose of RDX. The particle size of RDX has a great influence on the combustion process of the modified TEGN propellant. The melting and decomposition process occurs mainly in the condensed phase with the RDX particle size of 8.5 μm and 45 μm. As the RDX particle size increased to 90 μm, some RDX particles separates from the basic body and decomposes in the gas phase, the propellant burning surface and quality combustion speeded increased and the regularity of combustion reduced.