The high-resolution PCB bare board images in online high-precision AOI detection system are up to 60000× 60000 or more. In order to improve processing speed, image contours extraction and DP polyline simplification are used to reduce the amount of processing data. Distance measure of point to segment is one of determinants which affect the efficiency of DP algorithm. A new method to compute distance is proposed. Firstly, a rotating coordinate is established based on the two endpoints of curve, in which the new coordinate value is computed for each point and used to divide the points into three zones and calculate distance, and Manhattan distance is adopted in zone I and III, perpendicular distance in zone II. Compared with Dan Sunday’s method, the proposed method takes full advantage of the computation result of the previous point, and the way to divide the points is more concise and efficient, the distance metric calculation amount for points in zone I and III basically keeps, but the amount for points in zone II which own highest proportion reduces significantly. Experimental results show that the improved distance measure method can improve the efficiency of DP polyline simplification algorithm for high resolution PCB bare board image contours.