Abstract: This article uses first by the Sobel operator determination picture edge approximate position, then uses three time transect interpolating function to carry on the interpolation computation to the gradation edge chart, enables the goal edge localization to achieve sub-pixel element level. Uses the threshold value iterative algorithm after the interpolation image to obtain the threshold value characteristic image again. The experiment indicated that this method can accurately locate object′s edge, to achieve sub-pixel accuracy, superior to the traditional edge detection methods.