Relativistic backward wave oscillator is a high-power microwave generator with great application potential. Since a large amount of waste electron are deposited on the collector, the temperature of the collector rises, which reduces the safety and stability of relativistic backward wave oscillator. The collector temperature characteristics are calculated by using the Fourier thermal conduction model and the phase change heat transfer model. A method to reduce the collector temperature and shorten the retardation time is obtained. The results show that the collector temperature, water temperature, and vapor volume fraction show periodic fluctuation. The collector temperature can be reduced by increasing the working pressure, increasing the volume flow rate, decreasing the channel width, and increasing the thermal conductivity of the collector material. The temperature response of the collector thermal conduction is synchronized with the thermal interaction in the nanosecond's order. There is a retardation time for the convection and phase change heat transfer. The convection heat transfer retardation time is about 5 ms, and the phase change heat transfer retardation time is about 10 ms. The thermal conductivity, channel diameter, and volume flow rate have great effects on the heat dissipation, so they are the major influence factors of retardation time.