The viscoelastic Poisson's ratio of solid propellant seriously affects the analytical accuracy of the structural integrity of the solid rocket motor. Based on the digital image correlation (DIC) method, a stress relaxation test and a uniaxial constant tensile rate test were carried out to measure the viscoelastic Poisson's ratio of the composite solid propellant according to the stress characteristics of solidification cooling and ignition pressurization conditions. The effects of time, temperature, longitudinal strain as well as tensile rate on the viscoelastic Poisson's ratio of propellant were studied. In this paper, the main curves of the temperature and strain rate of the propellant Poisson's ratio at the reference temperature of 25 degrees C and the main curves of temperature and longitudinal strain at the reference time of 10000 s were established. The research shows that the viscoelastic Poisson's ratio of composite solid propellants increased with increasing temperature, longitudinal strain and tensile rate. Under the solidification cooling condition, the Poisson's ratio of the solid propellant does not change significantly with time. At this point, the Poisson's ratio can be regarded as a constant, but the appropriate value must be selected according to the operating temperature and initial deformation of the solid rocket motor. In the uniaxial constant tensile rate test, when the stretching rate reached 500 mm/min, Poisson's ratio of the propellant gradually approached a certain constant value. Considering that the loading rate of the propellant is generally above 0.5 s(-1) under the ignition pressurization condition, the strain rate is large, the Poisson's ratio of the propellant can be taken as a constant. Relevant methods and conclusions can provide a reference for structural analysis and storage life prediction of solid rocket motors.