The invention discloses a length detection device of hardware round tube finished products. The length detection device comprises a limiting frame and a conveying table, wherein the conveying table comprises a machine frame and a conveying belt set, a positioning and flattening device is arranged on the conveying table, the positioning and flattening device comprises a flattening plate, a verticaladjusting plate, a transverse adjusting plate and an auxiliary side plate, a positioning side plate is arranged opposite to the auxiliary side plate, a first waste box is arranged above the flattening plate, a pushing air cylinder is arranged at the bottom of the first waste box, the pushing air cylinder is connected with an intercepting frame, and a material blocking device is arranged on the rear side of the intercepting frame. The length detection device is simple in structure, high in practicability, good in safety, and convenient to operate; and automatic detection and classification canbe carried out on the length of the finished round tube finished products, and the round tubes with the length larger than the set range or smaller than the set range are taken away, so that the length of the round tubes collected by finished products meets the actual requirements, the operation automation is realized, the labor intensity of workers is effectively reduced, and the accuracy of thedetection and classification of the length of the round tubes is improved.