Aiming at dealing with the problem of digital detection of flush and gap between aircraft skins, the measurement technology of detection method is studied based on the grating projection technology. Firstly, the dense point cloud data of surface to be measured is obtained using the technology of grating projection; Then, the seam area is positioned from the image, and the point cloud data of the seam area is obtained according to the correspondence between the point cloud and the image; After that, the point cloud data is analyzed and the joint on both sides of a straight line segment is determined. And based on them, the flush and gap are calculated. Compared to the structure line technology, the proposed method can obtain more data. Besides, we can analyze all seams during one measurement and thus, the detection efficiency is very high. The experimental results show that the mean error of the proposed method is less than 0.03mm and the maximum error is less than 0.05mm, which can meet the requirements of aircraft skin seam testing.