Effects of the varieties and contents of nitrocellulose（NC）ball,particle sizes of ammonium perchlorate（AP）,contents of AP and RDX on the mechanical properties of casting high energy composite modified double-base (CMDB) propellant,were studied. Results show that,the tensile strength and elongation at +20 ℃ and +50 ℃ of the CMDB propellant decrease significantly with the decreasing of NC (12.0%N) content. Appropriate content of NC(13.0%N) and NC(12.6%N ) balls in CMDB propellant can enhance the tensile strength of CMDB propellant. But NC(13.0N%) ball is not helpful to the improvement of tensile elongation of the propellant,while NC(12.6%N) ball is beneficial to the improvement of the propellant elongation. At the same time,appropriate content of DB ball composed of NC(12.0%N) and NG in the propellant will improve the tensile strength and the elongation of the propellant. Particle sizes of AP have influences on the mechanical properties of the propellant. AP with smaller particle size is helpful to improve the tensile strength of the propellant, while AP with bigger granularity is conducive to the improvement of the propellant elongation. When RDX content in CMDB propellant increases gradually，the tensile strength of CMDB propellant at +20 ℃ and +50 ℃ firstly increases and then decreases,while the elongation at +20 ℃ and +50 ℃ firstly decreases and then increases.