In order to improve the measurement accuracy of planar sheet metal parts when the measurement scales increase, upper and lower edge distinguishing method and thickness converting model were proposed. Firstly, edge distribution characteristics of parts image which include upper edges and lower edges were proposed by analyzing the edge feature of different thickness of metal parts placed in back-lighting. Sub-pixel edges of parts were extracted by the Canny and polynomial fitting algorithm. Then, upper or lower edges were distinguished by distance-mean method. Finally, accurate edges on the calibration plane were located basing on thickness converting model. Dimensional measuring of metal part could be calculated using the calibration data. Experimental result shows that the precision of the 1-5 mm thickness and 500 mm×500 mm parts in the range of 1×0.75 m2 is up to 0.05 mm. It can satisfy the requirement of the sheet metal cutting measurement.