DEM generated by topographic map vectorization is wide used in many applications. Because the vague and adhesion phenomena in scanned terrain image will inevitably exist, the results of the contour extracting from blur images are not satisfactory in most cases. Combining with morphological mothod in both RGB and HSV color space, several elimination mothods are used to remove non-contour information, and the remains are color contour with many breakpoints. The UnSharpen Model(USM) is used to sharpen the contour line,then we use models to decrease the line width and to deblur the line in gray modes. By using the removed non-contour information, we propose a new breakpoint-matching method to connent the contour and generate more ideal binary contour with high-quality. The whole preprocessing algorithm is simple, easy to implement.