The invention relates to microwave source devices in the technical field of high-power microwaves and provides a wave-band-cross mechanical frequency modulation relativity back wave oscillator. The wave-band-cross mechanical frequency modulation relativity back wave oscillator comprises a cathode seat, a cathode, an anode outer cylinder, a cut-off neck, a slow wave structure, an inner conductor, a collector, a microwave output and a solenoidal magnetic field. The inner conductor is a cylinder, by adjusting the length of the inner conductor, the frequency of outputting microwaves can be adjusted, and outputting of wave-band-cross microwaves is achieved. An annular groove is formed in the collector, and a supporting rod is fixed on the anode outer cylinder. The wave-band-cross mechanical frequency modulation relativity back wave oscillator has the advantages that by adjusting the length of the inner conductor, frequency can jump between L wavebands and S wavebands, the adjusting manner is simple, the frequency in the wavebands can be adjusted, the bandwidth in the L-waveband frequency is adjusted by 7% or so, and the bandwidth in the S-waveband frequency is adjusted by 2% or so. The outer radius of a slow-wave vane can have a low value, and miniaturization is achieved. A groove structure is reserved at the left end of the collector, reflection of the tail end of the device can be increased, and achieving long-pulse outputting of the wavebands is facilitated.