The invention discloses a length detection device. The length detection device comprises a rack, wherein a feeding module is arranged on the front part of the upper surface of the rack; a loading module is arranged at the right side of the feeding module; a detection module is arranged behind the loading module; a non-defective product unloading module is arranged behind the detection module; a defective product unloading port is formed in the left side of an unloading module; a jacking module is arranged at the right side of the defective product unloading port; the feeding module is used forcontinuous feeding; the loading module is used for continuous feeding for a whole group; the detection module is capable of carrying out accurate detection for a single product; the non-defective product unloading module is used for collecting non-defective products; and the jacking module is used for jacking out defective products, and the defective products fall off from the defective product unloading port and enter a defective product collection container. In conclusion, the length detection device disclosed by the invention has the advantages of being high in detection efficiency, capable of lowering the labor intensity of workers, and accurate in detection result.