Based on the principle that the distance from the axle of an inspected long shaft to the two laser planes is unique, the method used image processing to get the coordinate of the cross point and the angle of the horizontal line of the laser image, and calculated the straightness parameters with distance position information along the long shaft. The image processing of the cross laser was carried out by a local maximum intensity. With a camera (640×480 pixels and 0.0973mm/pixel as resolution of target), 50 samples were taken at each distance of 1m, 9m and 18m. The standard deviation was 0.00937mm, 0.0342mm and 0.0860mm for polar radius, 0.843 ' , 1.26 ' and 1.57 ' for angle, respectively. Experimental results indicate that the straightness inspection system for long shaft is simple in structure, easy to operate and precise in inspection results.