The reflectivity of optical mirrors can be obtained by measuring linewidth of transmission spectrum of optical cavity and the fineness of Fabry-Perot (F-P) cavity. It is difficult for a spectrometer to measure the linewidth accurately due to its limited resolution. In this paper, laser phase modulation is introduced on the basis of fineness of F-P cavity, and laser phase modulation and fineness of F-P are proposed for measuring the reflectivity of optical mirrors. The frequency interval between the carrier and one sideband generated by electro-optical modulation is used as a radio frequency ruler to precisely measure the linewidth. Thus the reflectivity of mirrors is obtained with uncertainty reaching the level of 10-4 by the relation of fineness of cavity, spectral linewidth and reflectivity of mirrors.