The invention provides an automatic folding mechanism which comprises a discharging cavity for containing small cutting pieces and a clamping assembly which is located in the discharging cavity and can turn over. The clamping assembly comprises a movable clamping plate and a fixed clamping plate which are stacked, a clamping cavity for clamping the small cutting pieces is formed between the movable clamping plate and the fixed clamping plate, and the movable clamping plate can move close to or away from the fixed clamping plate. Some cutting pieces obtained after edge folding can be driven bythe turnover clamping assembly to turn over, an angle is formed, and therefore when large cutting pieces are placed, sleeve placket forming parts on the large cutting pieces can be located between themovable clamping plate and a small part of small cutting pieces not clamped in the clamping assembly, then, a small part of small cutting pieces are driven by the clamping assembly to continue to turn over so that the sleeve placket forming parts on the large cutting pieces can be clamped into the small cutting pieces, and the sleeve placket sewing efficiency and quality are improved.