In capacitive pulsed power supply (CPPS), semiconductors, such as crowbar diodes, can be damaged by surge energy in worst cases, which reduces the reliability of CPPS. The generation mechanism of surge energy is discussed initially. A surgesuppression scheme is presented to reduce the breakdown risks of semiconductors. Surge voltages and currents are simulated and analyzed based on the classical circuit models of CPPS and railgun. Then, two installation positions of the surgesuppression device are systematically investigated. The simulated results show that installing the surge suppression device at the railgun muzzle can suppress the surge voltages induced by an armature leaving the railgun barrel, and installing the surge suppression device at the railgun breech can reduce overcurrents of semiconductors in reverse recovery phases. Ultimately, experimental results validate the theoretical analysis.