A novel method for purging the residual hot burned gas in the cyclic operation of a pulse detonation combustor (PDC) was developed. With this new method, called the liquid-purge (LIP) method, residual hot burned gas is purged by injecting liquid droplets into the PDC. The injected liquid droplets are rapidly vaporized after the combustion of the detonable mixture. The vaporization of liquid is an endothermic phase transition accompanied by a roughly thousand-fold volume expansion. Consequently, the residual hot burned gas is cooled and pushed outward-in other words, purged. The accuracy of the injection of liquid droplets is controlled with a high-performance automotive fuel injector, enabling high-frequency operation of a PDC. In demonstration experiments, a PDC was successfully operated at the frequency of up to 350 Hz, using ethylene as fuel and pure oxygen as the oxidizer. These were supplied to the PDC in the valveless mode, and liquid water was used for the purge. In the analysis of the results, the hypothetical thickness of the water-vapor layer created in the vicinity of the closed end of the PDC was evaluated. This hypothetical thickness is proportional to the injected water mass per unit cross-sectional area of the PDC, and is a scale length of the separation distance between the residual hot burned gas and the fresh fuel and oxidizer. The results suggested that the hypothetical thickness of the water-vapor layer was an important governing parameter of the purge process.