Since a straightness inspection system for long shaft needs line and angle datum,a method of straightness inspection using cross laser for long shaft is proposed in this paper.Based on the principle that the distance from the axle centre of inspected long shaft to the two laser planes is unique,the image processing is used in the method to get the coordinate of the cross point and the angle of the horizontal line of the laser image and calculate the straightness parameters with distance information.The structure of inspection system is discussed.The calculation method for parameters of straightness is explained in detail.With a camera (640pixel×480pixel and 0.097 3mm/pixel as resolution of target),50 samples are taken at each distance of 1m,9m and 18m.The standard deviation is 0.009 37mm,0.034 2mm and 0.086 0mm for polar radius,0.843′,1.26′ and 1.57′ for angle,respectively.Experimental results show that the straightness inspection system for long shaft using the method is simple in structure,easy in operation and precise in inspection.