Effect of contents of plastic nitrocellulose(PNC)on mechanical property and preparation process of casting high energy composite modified doublebase (CMDB) propellant with low smoke were studied by means of HAAKE rheometer and single axes tensile test,and physical crosslinking density(νp) of the CMDB propellants were calculated by means of the B.S.T equation. Results show that the rheology of the CMDB propellants possess characteristics of pseudoplastic liquid,and viscosity (shear rate is 1 s^-1)of the CMDB propellant increases significantly with increase in content of PNC. The leveling property of the CMDB propellant becomes bad with increase in content of PNC; the pot life of the CMDB propellants with various PNC contents are long enough to meet the preparation process of the propellant. It is also found that,appropriate increase in content of PNC is helpful to enhancement of not only the tensile strength at +20 ℃ and +50 ℃ of the propellants，but also physical crosslinking density(νp) of the propellants； and it can also improve continuity and closegrained property of the propellants,and packing effect between binder system and particles.