The invention relates to a cooling device for a travelling-wave tube amplifier. The cooling device comprises a first cooling plate, a second cooling plate and a third cooling plate, wherein the first cooling plate, the second cooling plate and the third cooling plate are all internally provided with cooling liquid runners, the cooling liquid runners in the first cooling plate, the second cooling plate and the third cooling plate are sequentially communicated so that a cooling liquid can sequentially pass through the first cooling plate, the second cooling plate and the third cooling plate, and the first cooling plate, the second cooling plate and the third cooling plate are sequentially used for cooling a high-voltage power supply of the amplifier, a power factor corrector (PFC) power supply and a travelling-wave tube. By the cooling device, the problems that the travelling-wave tube amplifier is instable in running, a component is thermally damaged and the service lifetime of a system is reduced caused by slow cooling speed and low cooling rate in an air-cooling mode are solved.