Zeeman modulation magnetic rotation laser spectroscopy (ZM-MRS) was first used to seperate the magneticc rotation signal (n +-n -) effectively from the laser background and the high order terms of magnetic rotation signal in visible region. It not only has very high sensitivity, high signal to noise ration and no absorption background and also can suppress the rotational spectrum with high quantum number for simplifying complex electronic spectra. In this paper the theoretical analysis of this technique is simulated by computer. The results are fitted well with the experiment. This spectrscopic method is valuable for measuring free radicals and helpful for assignment of molecular spectrum.