The invention discloses a concave clearance adjusting device, a threshing device and a harvester. The concave clearance adjusting device comprises a power mechanism and a reversing mechanism, wherein the power mechanism reciprocates in the preset motion direction and controls a movable concave around the outer side of a roller to get close or away from the roller; the reversing mechanism is connected with the power mechanism and the movable concave separately, so that a first preset angle is formed between the motion direction of the movable concave and the motion direction of the power mechanism. Free adjustment of the movable concave clearance (namely, the distance between the movable concave and the roller) is achieved, the concave clearance meets the best threshing conditions of grain of different types and/or different water contents, and therefore the grain threshing quality is improved; the concave clearance adjusting device is simple and compact in structure, small in occupied space, easy to operate, has the good practicability and wide in application range (axial-flow concave or tangential-flow concave), and has the good market prospect and market competiveness.