干涉仪参考镜面形离焦误差难以精确标定，是导致拼接累积效应、制约平面子孔径拼接系统检测精度的主要因素。推导了参考镜离焦与拼接累积误差、拼接次数间的定量表达式，基于该表达式在拼接过程中标定并去除参考镜离焦误差，降低拼接累积误差。对450 mm×60 mm 的平面镜进行了8个子孔径的拼接检测，与大口径干涉仪检测结果比对，去除参考面离焦误差前后拼接测量误差峰谷(PV)值从λ/10减小至λ/30，有效提高了拼接测量精度。结合绝对检验技术标定参考镜高阶面形误差，验证了离焦是引起拼接误差累积的主要因素，消除参考镜高阶面形误差并不能显著提高拼接检测精度。

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.