A twodimensional axisymmetrical numerical flow model with secondary combustion and trail cover moving boundary is established to study the influence of volume change in initial chamber on the load and the internal ballistic property. On the basis of experimental data verifying the reliability of model, the mechanism of flow field and the secondary combustion core region change caused by volume change of initial chamber is investigated, and the dominant factor influencing building pressure in the rejection process is analyzed, and flow field, load and internal ballistic characteristics of different initial volume are obtained. The results show that with the increase of height in the initial chamber, the reflection point of jet flow is transferred from the cylinder bottom to the cylinder wall, and the secondary combustion core region is transferred from the upper part of cylinder to the lower part. For the initial pressurepeak, volume factor is leading, and for the second pressurepeak, the total pressure is leading. The peak values of missile acceleration and the outlet velocity decreases firstly and then increases, and the time out of tube becomes longer, and then becomes the shorter. The height of the initial chamber of the experimental device increases by 100mm, which is one of the most optimal interior ballistic designs.