The welding surface defect is an important index for evaluating welding quality. This paper designs a welding surface defect detection system based on the principle of optical coherent ranging to solve the problem of low measurement accuracy of the existing detection methods. First, based on the principle of the Michelson interferometer, a broadband laser is used to scan a point on the welding surface to collect the interference light intensity of the reference and measurement arms. Second, the interference spectrum corresponding to the scanned point is determined using the collected light intensity. Finally, the optical path difference corresponding to the spectrum is calculated to determine the axial height of the scanned point relative to the reference mirror. The experimental results show that the axial measurement accuracy and the lateral resolution of the system can reach 10 μm and 13 μm, respectively. Moreover, it can accurately identify surface pores, lack of welds, welding seam misalignment, and other welding defects and accurately measure the geometry and determine the shape of the weld.