The invention provides a wideband coaxial resonant cavity. The wideband coaxial resonant cavity comprises an outer conductor, an inner conductor, an upper end cover and a lower end cover, wherein the inner conductor is coaxially arranged in the outer conductor, the upper end cover is arranged above the outer conductor, the lower end cover is arranged at the bottom of the outer conductor, the inner conductor, the outer conductor, the upper end cover and the lower end cover form a coaxial resonant cavity, two through holes are symmetrically formed in a lower end of a side wall of the outer conductor around a central axis of the resonant cavity, coupling rings are inserted to the through holes, a plurality of groove bodies are circumferentially and uniformly arranged on an outer surface of the side wall of the outer conductor and vertically extend towards the center of the resonant cavity, the depth of each groove body is smaller than the thickness of the side wall of the outer conductor, a gap is arranged in each groove body and extends to an inner wall of the outer conductor, the width of the gap is smaller than that of the groove body, and the interior of the groove body is filled with a wave absorption material covering the gap. With the adoption of the abovementioned scheme, a large of parasitic modes can be effectively suppressed, a main mode is easier to recognize, particularly, the effect is more obvious in a relatively high frequency range, the frequency spectrum of a relatively pure TEM00p mode in a relatively wide frequency band range of 0.6-13.5GHz can be finally obtained, and more than ten discrete frequency points are covered.