Abstract: Laser welding process has demanding requirements on quality of butt joint preparation, especially on joint gap width and mismatch, the butt joint gap being narrow and little mismatch.Due to the inadequate lateral resolution or measuring principle limitation, current vision sensors can not measure the gap width accurately.In this paper, a vision system of joint gap width measurement is designed using the CCD camera with telecentric lens and diode laser, and a joint gap detection algorithm is proposed based on gray projecting integral approach.The experimental results demonstrate that the gap width measurement system can extract the joint border and joint gap width accurately;joint gap detection algorithm has self-verification ability to a certain extent.Accuracy of the measurement is better than 0.015 mm for a butt joint specimen with 0.05 mm gap width.