With the help of REAL software, the energy characteristics of composite modified double-based propellant containing different ratio of CL-20, RDX and Al were calculated using least free energy method. The effect of hexanitrohexaazaisowurtzitane(CL-20), RDX and Al content changing on the energetic characteristics of the propellant was studied. The results show that with increasing the CL-20 content, the theoretical specific impulse, combustion temperature, oxygen balance and average molecular weight of combustion gas increase significantly. With adding more aluminum, the energetic parameters of the propellant increase, but Al content should be suitable to ensure the coefficient of oxygen balance higher than 0.53, otherwise energy parameters of the propellant will be reduced.