The color mark recognition is a key step in Mirosot, the recognizing results affect the orientation angle to a great extent. During the capturing process there are some loss of color mark because of the environmental disturbing light, which can lead to a big error by calculation. For the above phenomenon, a new algorithm is designed based on the LSM (least square method), in which a circle is adopted to approximate the edge of the color mark after the pretreatment of the image for extracting the edge of color mark, so as to detect the center and calculate the orientation angle. It has been proved the recognizing precision and the calculating speed can be improved greatly through the experiment.