Non-linear errors resulted from mis-installation of common-path heterodyne interferometer affect the precision of its measurement system. Four factors which can lead to such non-linear errors were identified,they are the frequency mixing, the phase jump of different metals to be measured, the tilt of the measured surface and NA of the aperture. Based on heterodyne and vector analysis theories, origins and modes of the frequency mixing error and phase jump error were analyzed. On the above basis, procedures to improve the accuracy of the measurement system were suggested. It was demonstrated that the precision of the laser heterodyne measurement system can be improved to nanometer orders.