A metamaterial high-power microwave source relates to the fields of vacuum electronic technology, particle physics, and accelerators, including: a cathode, a metamaterial slow-wave structure (SWS), a waveguide and coaxial line coupler located at one end of the metamaterial SWS and a collector component located at the other end of the metamaterial SWS. The metamaterial SWS provided by the present invention is greatly smaller than a rectangular waveguide having the same frequency, so as to realize a miniaturization of devices and facilitate integration with semiconductor devices. The waveguide and coaxial line coupler has a good transmission characteristic and a low reflection in a relatively wide frequency band, which guarantees a high-efficient coupling output of a signal. Moreover, the metamaterial high-power microwave source has a high-power output and a pulsed output power reaching a megawatt level.