To effectively evaluate the aging invalidation of composite solid rocket propellant, it is necessary to set up the relation between the structure state parameter and the mechanical capability of composite solid rocket propellant. The rational relation model on them was built by applying the neural net. The simulation calculation on this model was done by combining the experimental data. The results show that the model can be used to study the relation between the structure state parameter and the mechanical capability of composite solid rocket propellant, and to evaluate the aging invalidation of composite solid rocket propellant.