To study the effect of hydroxyl terminated polybutadiene(HTPB) propellant with initial defects on its macroscopic mechanical properties, the multi-step relaxation and uniaxial tensile tests of the home-made HTPB propellants with different content of initial interface defect were conducted and the equilibrium response curve and tensile curves of HTPB propellant were obtained. The equilibrium response curve of HTPB propellant without initial defects was fitted by Ogden model. The strain rate parameter M was introduced to discribe the rate dependence characteristics of uniaxial tension curves of HTPB propellant, and the parameters of the visco-hyperelastic constitutive model of HTPB propellant without defects were obtained through fitting the curves. The influence of initial defects was taken into consideration, the visco-hyperelastic constitutive model of HTPB propellants with initial interface defects was constructed by introducting the initial damage factor f and all model parameters were obtained by fitting step by step. Finally, the macroscopic mechanical behavior of HTPB propellant under uniaxial tension loading was predicted by the model established by this investigation. Results show that the predicted results are consistent with the experimental ones, the maximum deviation between the two is only 4.4%, which confirms the reliability of the model.