Corresponding to the different gradient of the ideal working current, the movement process of the armature in the railgun bore can be divided into rising phase, stable phase and decreasing phase. Based on the load characteristic and the lumped-element equivalent circuit of the railgun, the characteristics of the railgun’s breech voltage in above three phases were analysed. Under the condition that the size of the launching system is close to the same, the change ranges of the breech voltage of the augmented railgun is much larger than that of the simple railgun, and the breech voltage of the augmented railgun is a very high forward voltage in the rising phase. The influence of the breech voltage on the discharge characteristics of the capacitive pulsed power supply was studied. In the rising phase, the high breech voltage of the augmented railgun has a significant adverse effect on the pulsed discharge characteristics of the capacitive pulsed power supply whose pulsed forming net consists of a large number of pulsed forming units with small energy storage and high resonance frequency. As a result, part of the stored energy of the capacitive pulsed power supply cannot be used effectively, which leads to the decrease of the working efficiency of the launching system. Using the launching system with a 13-MJ capacitive pulsed power supply and a medium calibre augmented railgun, the launching simulation and test were completed, and the analysis was verified. The research has important enlightenment for improving the match between the capacitive pulsed power supply and the railgun.