To improve the structural integrity of solid rocket motor grain with large aspect ratio, based on three-dimensional viscoelastic finite element method and taking into account of the temperature load and pressure load, the structural response of a solid rocket motor with large aspect ratio was analyzed, the effects of the umbrella slot and its location on maximum Von Mises strain of the grain were studied, and the variations of the maximum Von Mises strain of the umbrella slot with its width, depth and radius ratio were discussed when the umbrella slot was in the middle of the grain. The results show that the Von Mises strain is reduced when the umbrella slot is in the middle of the grain; increasing the width, depth or choosing the best radius ratio can reduce the maximum Von Mises strain of the umbrella slot. The conclusions are beneficial for designing the grain configuration of solid rocket motor.