Optical heterodyne Zeeman modulation magnetic rotation spectroscopy (OHZM-MRS) is a high sensitive spectroscopy technique. In this spectroscopy, both laser frequency and Zeeman effect of a sample are modulated, and then the signal with high signal-to-noise will be obtained by double demodulation. The expression of OHZM-MRS signal is derived, and the influences on the line shape of OHZM-MRS are discussed by the extinction coefficient of polarizers, ellipticity, phase shift induced by elliptical polarization, φ angle (where φ is the small uncrossing angle between two nearly crossed polarizers) and residual amplitude modulation. The results of theory and experiment are in agreement each other.