1. A system for scanning edges of a workpiece, the system comprising an image measuring machine and a computer, the image measuring machine comprising a charge coupled device (CCD), a work platform configured with the workpiece ready to be scanned, and three motors, the computer comprising: a parameter setting unit configured for setting parameters of scanning edges of the workpiece; a motion control adapter card connected with the motors, and configured for controlling movements of the motors to locate the work platform and focus the CCD on the workpiece; an image adapter card connected with the CCD, and configured for collecting at least one image of the workpiece captured by the CCD; an image obtaining unit configured for obtaining an image from the image adapter card if the CCD captures the workpiece; an image processing unit configured for executing an image sharpness process and an image binarization process in order that the background color of the image is opposite to the color of the workpiece on the image; an edge extracting unit configured for searching a first edge point near a designated point in the image in a clockwise direction, wherein there is at least one point adjacent to the first edge point whose color is different with the color of the first edge point, and configured for searching other edge points of the image along with the manner of searching the first edge point by utilizing a recursive method, and extracting an outline of the image based on the searched edge points; an image edge scan unit configured for calculating normal vectors and tangent vectors for each edge point on the outline, calculating coordinates of initial points and end points of scan lines according to the corresponding normal vectors and the parameters, and searching scanned points on the image that has not been processed accurately according to the initial points and the end points, and scanning the edges of the workpiece based on the scanned points; a detecting unit configured for detecting whether each of the scanned points is within the image, and detecting whether all the scanned points collectively form a closed edge path of the workpiece; and a calculating unit configured for calculating a move distance of the work platform and determining the move direction of the work platform according to the tangent vectors, in order to capture a next image of the workpiece if any scanned point is not within the image or if all the scanned points collectively form an open edge path.