Aiming at the problem that the imaging accuracy of the planar part was decreased due to pseudo edges in the illumination of the backlight, a measurement method for recovering the accurate edge from the pseudo edges of the measured object was proposed. Firstly, pseudo sub-pixel edges of planar parts were extracted by Canny algorithm and surface fitting algorithm. Secondly, upper and lower edge points distinguishing method based on direction angle difference was proposed and the upper edges were restored by the method of mean Euclidean distance. Finally, the upper edges were converted to the measurement plane by the similar triangular method, and dimension of planar parts could be calculated by using the camera calibration parameters. The results show that the absolute error is less than 0.08 mm and the relative error is less than 0.05% in the range of 1 200 mm×800 mm.