Influence of different ratios of 1,3,5-trinitrohexahydrotriazine(RDX) to 3,4-bisnitrofurazan-furoxan(DNTF) on the mechanical properties of DNTF/RDX composite modified double-base(CMDB) propellant (DNTF/RDX-CMDB, DFR propellant) was studied by uniaxial tensile test, charpy impact test and dynamic mechanical analyzer.The tensile strength (σ_m), elongation at the maximum stress(ε_m) and impact strength(a_K) of DFR propellant at -40 ℃ were obtained. The dynamical characteristics (viscoelastic coefficients C₁^g) of β -relaxation was acquired by WLF equation according to the "Time-Temperature" equivalence principle. The effect of plasticizer content on the various mechanical properties and change rule of σ_m and ε_m were analyzed, showing that a good correlation between σ_m, ε_m, a_K at low temperature and plasticizer content. The plasticizer content reducing, C₁^g increasing, "free volume of side group of NC molecule" decreasing make the mechanical performances (σ_m and ε_m) worsen.