Sub-aperture stitching is an effective way to extend the lateral and vertical dynamic range of a conventional interferometer. The test accuracy can be achieved by removing the error of reference surface by the absolute testing method. When the testing accuracy (repeatability and reproducibility) is close to 1nm, in addition to the reference surface, other factors will also affect the measuring accuracy such as environment, zoom magnification, stitching precision, tooling and fixture, the characteristics of optical materials and so on. In the thousand level cleanroom, we establish a good environment system. Long time stability, temperature controlled at 22°±0.02°.The humidity and noise are controlled in a certain range. We establish a stitching system in the clean room. The vibration testing system is used to test the vibration. The air pressure testing system is also used. In the motion system, we control the tilt error no more than 4 second to reduce the error. The angle error can be tested by the autocollimator and double grating reading head.