The uncertainty of parameters may greatly affect structural analysis of solid propellant grain. Nonpropabilistic convex set theoretic models, combining viscoelastic finite element method, are developed to study the uncertainty of structures. Incremental method is applied to solving the hereditary integrals. Perturbation method is used to predict the variability of structural response. The developed method is suitable for uncertain structural analysis of solid propellant grain. By comparing with stochastic structural analysis, relations between the two methods have been found.