This paper describes a method that can correct systematic errors in the subaperture stitching measurement after analyzing the absolute test of three-flat testing. The testing gets a vertical profile of data and horizontal profile of data along the diameter of the reference flat, and then using the data to fit an error correction wavefront of the reference flat, namely the use of Zernike polynomials (defocus and ast) for lowlevel fitting. Experiments show that this method can eliminate the systematic errors caused by the interferometer reference flat, and effectively improve the accuracy of wavefront stitching.