<p>As a core component, the mechanical behavior and failure criterion of solid propellant grain are the key to environmental adaptability and safety of solid rocket motors. However, considering variable Poisson’s ratio with significant strain dependence as an elastic constant and failure behavior with loading time dependence as fixed limits are important factors in the current inaccuracy of motor structural integrity analyses. Here, a series of constant speed tensile and creep tests were carried out. The variable Poisson’s ratio phenomenon and time-dependent failure behavior of propellant were analyzed and discussed, and the microscopic mechanism was revealed. A model for predicting time-dependent failure behavior of composite materials is proposed, and each model parameter has a corresponding macroscopic and microscopic physical meaning. The validity of model prediction accuracy is verified in three working conditions, and the coefficient of determination R² is between 0.88 and 0.98. This work could provide theoretical and experimental guidance for long-term service life prediction and reliability evaluation of solid propellant, as well as other particle-reinforced materials.</p>