The invention discloses a compact relativity backward wave oscillator (RBWO) with an adjustable low-frequency-range frequency, and the oscillator provided by the invention is used for solving the problems that the RBWO size in a low frequency range is great, and the output microwave frequency is hard to adjust. The compact RBWO is in rotational symmetry with respect to a central axis, and composed of a cathode base, a cathode, an anode outer barrel, a stop neck, a slow wave structure, an inner conductor, a collector, a microwave output port, a solenoid field, two rows of supporting rods, a mode converter, a radiation port and a sealing plate; the slow wave structure is composed of five slow wave blades, the inner surface of each slow wave blade is in a trapezoid structure; the left end face of the collector is provided with an annular groove; the left end of the mode converter is cylindrical, and the right end of the mode converter is in a tapered structure; the radiation port is cylindrical, the left end of the radiation port is in a tapered structure and the right end of the radiation port is cylindrical; the sealing plate is pressed on the radiation port; and the frequency of the output microwave is adjusted by virtue of adjusting the semi-diameter R3 of the inner conductor. The compact RBWO has the advantages of compact structure and convenient and adjustable work frequency, thereby being beneficial to outputting a long pulse of the microwave.