The invention discloses a threshing mechanism on a corn thresher. Walls of a first threshing chamber and a second threshing chamber are fixed on a frame, threshing cylinders arranged in the threshing chamber are rotationally connected with the frame, cylinder teeth are fixed on the threshing cylinders in a stagger arrangement manner, a discharge outlet arranged at one end of the first threshing chamber is communicated with one end of the second threshing chamber, a suction fan is arranged on the upper portion of the discharge outlet of the first threshing chamber, and the cylinder teeth on the first threshing cylinder located at the discharge outlet of the first threshing chamber are higher than those, not located at the discharge outlet of the first threshing chamber, on the first threshing cylinder. The middle cylinder teeth, not at the discharge outlet of the first threshing chamber, on the first threshing cylinder are higher than those on two sides. The top ends of the cylinder teeth at the upper portion of the first threshing cylinder are higher than the top ends of the cylinder teeth on the upper portion of the second threshing cylinder. The threshing mechanism has the technical effect that cleanliness of corn grain is improved.