The three-dimensional linear viscoelastic model was used for analyzing the influence of number m and length-diameter ratio on structural integrity of solid propellant grains under axial overload. Nine different numbers m and six length-diameter ratios for fifty four models were selected to measure effect on structural integrity. The results shows that number m and length diameter ratio have cooperative effect on the maximum value of equivalent von Mises stress and strain. If length-diameter is more than 3, it has less influence on the maximum value of equivalent von Mises stress and strain, and it can be referred to the selection of number m and length-diameter in structure design of solid propellant grain.