Abstract: With a mechanical length-specifying system, the old type pneumatic flying-saw for length-cutting of welded pipes had such disadvantages as high faliture rate and low cut-length accuracy. Accordingly, modification has been conducted to the said saw system with its mechanical and pneumatic parts unchanged. The modification includes measures, namely, using solid-state relay to drive the solenoid valve directly, measuring pipe length via site pulse counter and feeding back of the measured length/speed data of the tube via length sensors to the microprocessor, which ensures the cut-length tolerance within 10mm.