3D image merging is a key technique in large scale free-form surface measurement.Based on image-control-point-restraint theory,a new 3D image merging method is presented.In the method,a pseudo-random space code is firstly projected on a large object surface,and the images of the object are taken from different angles of view.Through matching the interest points on the overlapping region,the coordinate transformation matrix is obtained and 3D measurement of large-scale free-form surface can be realized.The experimental results show that this new image merging method by space code projection can measure large-scale 3D shape and the relative error of merging data is less than 0.8%.