To study the biaxial compressive mechanical properties of solid propellantn， it is necessary to determine the optimal propellant specimen configuration. This configuration should be compatible with the testing machine and test fixture and meetthe requirements of biaxial deformation characteristics. Based on the finite element numerical simulation calculation， the deformation stress contour of the three-component HTPB composite solid propellant specimens with eight different configurations under biaxial compression loading were obtained. Moreover， the optimal propellant specimen configuration was verified by conducting the dynamic biaxial compressive mechanical properties test on the corresponding specimen. Results show that the stress contour of all specimens under small deformation （strain within 10%） is uniform overall. However， the requirement of the plane stress does not meet during deformation of the specimens with an aspect ratio greater than 1. Furthermore， the average value of plane stress， dispersion of plane stress， the whole stress stability factor and the stress concentration factor were selected as the optimizing objective function of the propellant specimen configuration. The contrastive analysis shows that the optimal configuration is a 25 mm cube. Finally， the validity of the above determinated optimal configuration was verified by analyzing the characteristics of stress-strain curves of the propellant specimens obtained under dynamic biaxial compressive loading condition