To improve the stability and consistency of base bleed unit while the projectile was out of the gun muzzle, the characteristics of transient combustion of composite base bleed charge was studied by using a semi-closed bomb to simulate a abrupt depressurization and a high speed digital video camera to record the phenomena of combustion. The pressure of combustion chamber in semi-closed bomb was controlled to vary from 20 MPa to 90 MPa, and the velocity of depressurization from 1.2×10³ MPa·s^-1 to 6×10³ MPa·s^-1. The results show that there are three kinds of combustion characteristics, e.g. reignition,extinguishment and critical case, when pressure and depressurization rate change. The depressurization rate and the pressure before depressurization are key factors influencing the combustion of base bleed charge. The analysis indicates that the cornbustion of base bleed charge is in the critical zone between extinguishment and reignition when the base bleed projectile is just out of the gun muzzle.