The invention provides a corn thresher which comprises a feed chamber capable of conducting screening, a size sorting chamber and threshing chambers. The feed chamber is arranged on the top of the size sorting chamber. At least two size sorting partition strip groups are vertically arranged in the size sorting chamber. Each size sorting partition strip group comprises at least two partition stripsin parallel in the horizontal direction. The distance between every two adjacent partition strips of the size sorting partition strip group on the upper side is larger than the distance between everytwo adjacent partition strips of the size sorting partition strip group on the lower side. The threshing chambers are arranged at the corresponding positions of the size sorting chamber. A threshingrotating shaft is arranged in each threshing chamber and provided with threshing teeth. The sizes of gaps between the threshing teeth in each threshing chamber and the inner wall of the threshing chamber are not the same. Compared with the prior art, the corn thresher can selectively thresh and process corns with different sizes with pertinence, and therefore the corn threshing effect is improved.