The power of reference flat (RF) in interferometer is difficult to be calibrated accurately. It leads to stitching error accumulation effect in sub-aperture stitching interferometer for flat optics, and becomes the major restriction factor for improving stitching accuracy. A quantitative equation is deduced for calculating the power of RF from stitching accumulation error and stitching numbers. Then the power of RF is calibrated and removed in the process of stitching. The error Accumulation is reduced. A flat mirror with aperture of 450 mm×60 mm is tested by 8 sub- apertures. Compared with the test result of a large aperture interferometer, the stitching measurement error is reduced from λ/10 peak-valley value to λ/30 PV with power compensation. The stitching test accuracy is improved effectively. Using absolute flatness test, the high order surface figure of the RF is also calibrated. It is verified that the power is the main source of the error accumulation in stitching. Removing the high order surface figure of the RF cannot improve the stitching accuracy significantly.