The invention discloses an ultra-wideband quasi-open slow-wave structure comprising two identical metal plates and supporting metal walls or mediums on the two sides. The center positions on the broad edges of the lower side of the upper metal plate and the upper side of the lower metal plate are each loaded with a striped metal ridge with periodic ups and downs along the longitudinal direction, wherein the width is Rw, the height is Rh, and the cycle length of the ups and downs is p. Thus, a striped electron beam channel is formed in the space between the striped metal ridges, and the height hb is the difference between the difference b of the metal plates and two times of ridge height Rh. The slow-wave structure has a natural electron beam channel, requires no additional processing, is of low processing difficulty and large coupling impedance, and is very beneficial to oscillation starting of a backward wave oscillator. The slow-wave structure is of low low-end cut-off frequency, and has the characteristics of ultra wideband and small high-frequency transmission reflection.