Abstract: The large-aperture interferometer can not be regarded as shift-invariant linear imaging systems since for it has different forms of point-spread function at different field positions. It is necessary to revise the calibration of system transfer function by the method of subaperture stitching. The large field is divided into many isoplanatic regions and the transfer function of each isoplanatic region is measured by standard phase step. Then the revised system transfer function can be expressed as the weight average of the transfer function of each isoplanatic region, whose weight depends on power spectrum of the actual test surface at the corresponding region. The experiment shows that the difference between the revised system transfer function and the zero-field transfer function is large