The most popular online high-precision Automatic Optical Inspection (AOI) detection system for PCB bare board adopts the inspection method based on contour vectorization and comparison, and arc detection is the key of realizing the contour vectorization. A new fast arc detection method is proposed. Firstly, segmentation primitives are obtained from contours by Douglas-Peucker polyline simplification algorithm and their normal angles are calculated.Then, the sum of difference between normal angles of each two adjacent primitives is calculated, which is used to build the normal angle transformation curve with the position of mid-point of primitive. Finally, DP algorithm is used again to detect the line segments in normal angle transformation curve to implement arc detection. The proposed algorithm simplifies the arc detection into two times line segment detection, and the complexity is O(n). Experimental results show that the proposed algorithm owns higher accuracy and lower cost time in comparison with the same module of Halcon software.