In order to acquire overall three-dimensional (3D) defect information, a new 3D defect detection method is designed. It only needs single image captured from real-time measurement line. From the single image, 3D data of the work piece shape ranged from 0° to 180° can be got. Moreover, it can give an overall recognition of plane dimensions and depth information of the defect. The key technique is made use of the 3D clues——“shading value”, which left in the single measurement image, to process luminance analysis and conversion, thus to calculate the depth information by slant and tilt. In the application of defect detection for magnetic work piece, the resolution in X and Y direction can reach 0.1 mm, and the resolution in Z direction can reach 0.007 mm. This experiment proves that the designed 3D defect detection method has the merits of simple operation, low cost, high speed and high precision. This 3D method is fit to use in industrial application.