The invention relates to a microwave component in the technical field of high power microwave and provides a coaxial-extraction long-pulse relativistic backward-wave oscillator. The coaxial-extraction long-pulse relativistic backward-wave oscillator comprises a cathode base, a cathode, an anode outer cylinder, a cut-off neck, a slow wave structure, a tapered waveguide, an output waveguide, a solenoidal field, a coaxial-extraction structure and a front-arranged reflection cavity. The coaxial-extraction structure is a cylinder. The front-arranged reflection cavity is disposed between the cut-off neck and the slow wave structure. According to the coaxial-extraction long-pulse relativistic backward-wave oscillator, a cylinder is adopted as the coaxial-extraction structure, and therefore the defect that the coaxial-extraction structure with a groove generates plasma is overcome, and meanwhile in the electromagnetic wave mode, function transformation can be carried out and installation can be achieved conveniently. The front-arranged reflection cavity is utilized to replace the cut-off neck so that the resonance characteristic of the front-arranged reflection cavity can be utilized to achieve the effect of the cut-off neck, electron beam scraping or electron beam bombardment of the front-arranged reflection cavity can be avoided, and premodulation of electron beams emitted from the cathode also can be carried out. Therefore, the coaxial-extraction long-pulse relativistic backward-wave oscillator is beneficial for subsequent beam wave interaction and improves power conversion efficiency of the component.