Residual amplitude modulation (RAM) in electro-optical phase modulators (EOM) results in unequal amplitude of modulation sidebands. Different amplitude attenuation of the first-order sidebands is undergone by means of transmission characteristic of Fabry-Pérot cavity and the Pound-Drever-Hall technique. Thus the amplitude of modulation sidebands is equal and the residual amplitude modulation can be suppressed. The beat signal between carrier and modulation sidebands is calculated theoretically after transmitting the Fabry-Pérot cavity, and the asymmetry of sidebands is reduced by four orders of magnitude once the Fabry-Pérot cavity is locked properly. Finally in experiment, the optical heterodyne spectrum is obtained after the modulated laser beam transmitted the Fabry-Pérot cavity. The result shows that the suppression of the residual amplitude modulation in electro-optical phase modulators is about 45 dB when the Fabry-Pérot cavity is locked to the central zero point of optical heterodyne spectrum.