Abstract: Through-the-wire sensor of the waveform-controlled short circuit CO₂ welding is studied in this paper.A system designed for wire extension signal measure and process is established based on the MCU (Motorola MC68HC11A1) after theoretical analysis of wire extension heat transfer and its resistance,molten drop profile and its resistance properties.During the period of short-circuit,the wire extension is approximate the distance from the welding torch to work piece,so that it can be used as the sensor for determining the height of welding torch,and for realizing the seam tracking.The processing results show that the short circuit voltage between the torch and work piece is in direct proportion to the wire extension,when welding current keeps constant during the medium-term of short circuit.The actual measure accuracy of the sthrough-the-wire sensor can reach &177;0.4 mm,therefore the requirements of torch height control and wire energy compensation control of CO₂ welding can be met perfectly.