In this paper,a scheme of a novel frequency synthesizer based on the optical heterodyne technique is proposed.Afrequency comb with frequency range from DC to hundreds of GHz can be obtained in the frequency synthesizer proposed by us.A 10 GHz-photoelectric detector is utilized to detect the multi-longitudinal-mode laser beam which is produced by a multi-longitudinal mode Er3+-doped fiber ring laser with cavity length of 27 m,and a series of beating signals with frequency spacing of 7.63 MHz are obtained at the output port of the detector.The frequency range of the frequency comb is from DC to 10 GHz restricted by the bandwidth of the photoelectric detector,which is 10 GHz.Frequency selection and combination can be realized based on the tunable band-pass filter.