Because of the limitation of the F-P interferometer for the analysis of a CW laser modulation spectrum,an optical beat frequency method is proposed,which can be used to measure the spectrum with a modulation frequency ~kHz. This method is based on the photoelectric conversion technique. We can get the beat frequency signal of the reference laser and a CW modulation spectrum with the spectrum analyzer,and then have the relative electric field strength of the various optical frequency of a CW laser modulation spectrum through the corresponding mathematical calculation so as to realize the CW modulation spectrum analysis. After theoretically modeling,experiments were carried out to measure a CW laser modulation spectrum generated by a diode laser and an EOM device with the proposed method. Compared with the measuring results with F-P interferometer,experimental results show the feasibility of an optical beat frequency method.