For parallelism measurement of large object, a parallelism measuring system which is composed of twodiffraction gratings is devised and established. The two gratings are placed orthogonal, and produce several parallel lines, which can be taken as baselines to measure the parallelism of the object. The picture taken by the camera is including the baselines and the object’s lines. Through this picture, it will get the coordinates of the baselines and the object’s lines. By the angle between the baselines, it can correct the angle between the object and the camera. By testing the angle of the large object in the new coordinate system, the parallelism of the object can be gotten. The result indicates that the method is simple with high accuracy.