Abstract: The weldment position in electro gas welding（EGW） is one of the important preparations to ensure the welding process. Using vehicle-mounted speakers and microphones to form a sound wave transmitting and receiving array,combined with the ranging principle and demodulation algorithm of frequency modulated continuous wave（FMCW） radar,the sound wave signal was processed, and a weldment position detection system based on FMCW frequency modulation ranging was designed and developed. The position image of the weldment was reconstructed, the average gray value of the region of interest（ROI） of the reconstructed image was calculated by sending different decibel acoustic signals, and the relationship between the average gray value of the reconstructed image ROI and different sound wave decibels was obtained, then the binarization of the reconstructed image and measurement of the gap between the unconnected areas were carried out, the relationship between the unconnected area in the image and the position of the weldment was established.The results show that the developed weldment position detection system has good practicability and accuracy in identifying the shape and position of the weldment.