A one-dimensional unsteady flow calculation model of a base bleed unit interior ballistics was established based on the theory of homogeneous flow and solid rocket engine. The inner flow field of base bleed unit and exterior trajectory of a base bleed projectile were simulated via calculating interior and exterior equations of base bleed projectile. The distributions of pressure, temperature, density and velocity of gas and their changing laws at nozzle were investigated and obtained. The calculational results show that the highest temperature and velocity of the gas are 1944 K and 65 m·s^-1. The working time of base bleed unit is 24 s and increasing range rate is about 30%. Both of working time and increasing range rate are in good agreement with the results obtained from practical shooting tests.