As the main energy source of artillery weapons, the design and selection of propellant charge determines the artillery power. The propellant grain is an important component of the propellant charge, and its shape is determined by the ballistic performance. The change of grain size will affect the launch safety of propellant charge. In order to study the influence of different propellant grain length on the launch safety. In this paper, the 130 mm gun is taken as the research object. Based on the interior ballistic two-phase flow dynamics model, the geometric burning law and the equivalent shape function, the influence of different grain length on the maximum intergranular compressive stress at projectile bottom in the interior ballistics initial stage is analyzed. The results show that when the other parameters such remain unchanged, the grain length changes in the range of −27%–41%, and the change rate of the maximum stress is not higher than 4%. This analysis provides theoretical guidance for optimizing base charge launch safety and offers a valuable reference for further launch safety research and charge structural improvements.