To solve the problem of line scale identification affected by line scale wear, line scale scratch, camera imaging out of focus, weak or strong exposure and uneven illumination of the line ruler, an algorithm for line scale identification and central line eigenvalue extraction is proposed. Firstly, the location of each gray extremum of line scale is acquired by region division method.Then,the eigenvalue of each center line of line scale is determined by extremum weighted method. Finally, the gray value on each center line of image is firstly smoothed, and then the gray jump terminal coordinate value of each line scale is searched, and the central line eigenvalue of the main line scale is derived according to the coordinate value. The experimental results show that the algorithm for line scale identification and central line eigenvalue extraction can eliminate the influence of the problems above,and the eigenvalue extraction of each central line is correct.