Research on key technology of the serial propellant charge has been done for the super high firing-rate barrage weapon in order to improve its interior ballistic performance. A quasi-two-phase flow interior ballistic model of mathematics and physics is established based on basic knowledge of interior ballistics and two-phase flow through reasonable hypothesis. A computing method is gained and a computing program is written by using MATLAB. Numerical solution is done on the basis of 30 mm caliber weapons and a better three-dimensional illustration on computing results is gained, the burning rules of gun propellant and process of projectile movement are reflected on the illustration. A calculation for the sealing and unsealing of side chamber is carried out theoretically. The results show that the muzzle velocity is raised from 588.4 m/s (unsealing) to 626.3 m/s (sealing) under the condition in keeping the maximum pressure basically unchanged. It means the sealing of side chamber could improve interior ballistic performance of the serial propellant charge in some extent.