Based on the wavefront aberration theory and interferometric theory, the influences of imaging distortion in an interferometer on measuring results are analyzed and calibration and correction methods for the imaging distortion are proposed. The measurement errors caused by misalignments of the test surface (tip/tilt and disfocus) are discussed mainly. An experiment is carried out to measure the same surface under different number of fringes. The test results show that the surface shapes are 30.96 nm PV and 6.32 nm RMS for 3 fringes, and 41.25 nm PV and 8.22 nm RMS for 10 fringes,respectively. After the calibration, the difference of the results are declined to 1.42 nm PV and 0.4 nm RMS. The experiment proves that the calibration solution referred in this paper can improve the measurement accuracy.