The invention discloses a length measuring device for steel pipes. The length measuring device for steel pipes comprises a long arm support (1), a guide rail (20), a support base, a push plate (4), a connecting box (3), a first bar (11), a second bar (12), a sensing block (10), a thick spring (14), a thin spring (15), and three proximity switches (21, 22 and 23). The support can be powered by a motor mounted on the support base to move back and forth along the guide rail. The proximity switches are all mounted on the connecting box. The sensing block is mounted on the second bar. When the sensing block moves to the first and second proximity switches, the motor on the support base stops rotating, and a motor encoder calculates a moving distance of the connecting box to obtain the length of a steel pipe. The length is measured by the aid of gear-rack drive, so that the length measuring device for steel pipes is suitable for measuring long pipes; an auxiliary gear is provided, and a power system can be more stable and durable; the length measuring device for steel pipes has the advantages of simple and practical structure, convenience of operation and accuracy of measurement.