Correlative stitching is on the fact that the same area has the same information. This testing thought is meaningful in extending spatial measurement ranges, keeping high resolutions, high precision and low cost. So in order to test large-scale optical workpiece, people are designing large-scale interferometer, at the same time, they are also designing stitching interferometer. The keys to realize stitching measurement are to obtain high precision wavefront of each sub-aperture and apply appropriate stitching algorithm. There are many techniques to test sub-apertures, among which phase-shifting technique has high precision, and is applied widely. How to reduce its system error is a central problem. The paper will utilize difference of two testing results to remove the system error. How to reduce the accumulative error is a key problem in stitching. The paper will apply the stitching algorithm in Descartes coordinates presented by M. Otsubo and K. Okada to realize the connecting of sub-apertures. And the paper presents a method to deal with the main random errors in sub-aperture testing. Finally, the paper does some tests.