A measuring method of large sized workpiece based on SIFT algorithm is proposed in this paper. After the different image matching algorithms are compared, SIFT algorithm, which has much more robust, is introduced in detail. Then measuring platform is built, and large sized workpiece is measured by SIFT image stitching and image processing. The experiment proves that SIFT algorithm is viable and advantageous in the measurement of large sized workpiece. At last, the precision and affecting factors are analyzed.