为了提高线偏振相移干涉检测的精度，分析了参考光和测试光的偏振方向正交性对该项检测技术的影响。分析了线偏振相移干涉的原理，提出偏振非正交引起的检测误差与波像差有关，且检测误差峰谷值与测试光和参考光振幅之比以及偏振非正交量成正比。讨论了偏振非正交产生的原因，认为测试光在被检元件表面的折射或者反射都有可能引起偏振非正交。针对不同被检元件，给出了减少偏振非正交的几种方法，包括合理镀膜、选择合理入射角偏振方向等。通过Skip-Flat检测对理论分析进行了验证。结果显示，对于入射角为45°的检测光路，偏振正交性引入的检测误差呈现与条纹一致的分布，峰谷值为0.174 1λ，这与理论分析相吻合。文章指出，进行偏振相移干涉检测时，需要针对不同的被检元件进行分析，以确保偏振非正交产生的检测误差能够满足技术要求。

The influence of polarization direction orthogonality on Polarization Phase Shifting Interferometry(PPSI) was analyzed to improve its measurement precision. Based on the analysis of PPSI principle, it points out that the measurement error is a function of Optical Path Difference(OPD) and the PV of measurement error is proportional to the non-orthogonality and the ratio of amplitudes of test beam to reference beam. The polarization non-orthogonality can be introduced by the reflection or refraction on a surface. Then several approaches were recommended to depress the non-orthogonality, such as proper coatings, incident angles and polarization directions. A configuration of Skip-Flat test with incident angle of 45° was set up. It shows that the distribution of measurement error is coincident with that of OPD. Moreover,the measurement error is 0.174 1λ (PV), which is in agreement with that of the theoretical analysis.Finally, the paper suggests that it is necessary to analyze individually each of the component under the test for making sure the measurement precision.