A novel algorithm is presented to compute the plane equation of line structured light, which used in a machine vision measurement system. Image subtraction is introduced to obtain stripe image, Steger algorithm is applied to extract sub-pixel level centric points of stripe in the image, and orthogonal line fitting is performed to gain line equation of stripe in image coordinate system. The approach of calculating world coordinates of points on the plane of line structured light from feature points in calibration board is introduced, which are based on the principle of cross-ratio invariance. Finally, orthogonal least-squares plane is proposed fitting to get light plane equation. Some specific optimizations in algorithm are put forward to improve total accuracy, and the scheme design of calibration system and detailed experimental procedure are given. Experimental results demonstrate that the method is of robustness and has high accuracy.