Nonlinear errors caused by mis-installation of common-path heterodyne interferometer affected the measurement precision of the system. Frequency mixing errors caused by the two factors existing at same time, which are elliptic polarization of laser source and installation azimuthal error of Wollaston prism, were analyzed especially in common path heterodyne interferometer. It was found that the nonlinear error is up to 2.2 nm and the error caused by the two factors may be counteracted each other a certain extent on some condition. The magnitude of error and the way of changing were given, which are of great importance in improving the measurement accuracy in common-path heterodyne interferometry. To the above basis, procedures to improve the accuracy of the measured system were suggested.