A new method for the automatic calculation of the low contrast laser spot center base on contour growing.The first step of the method is automatic segmentation,then divide noise and ghost reflections from the laser spot by contour growing,in the end a circle is adopted to approximate the edge of the laser spot by the contours which are similar to a circle.Comparative tests shows that the method is more accuracy and more automatic than the previous method.The accuracy of method meets the requirements of the SGⅢ prototype facility．