The present invention provides a tunable optical frequency converter based on a phase modulator, which comprises a laser, a first optical isolator and a circulating frequency shift module. Said circulating frequency shift module is composed of an optical coupler, a phase modulator, an optical amplifier, a second optical isolator, a first optical circulator, an optical fiber Bragg grating, a second optical circulator, a tunable filter and a tunable attenuator. Light outputted by said laser is inputted to said circulating frequency shift module to conduct frequency converting repeatedly after passing through the first optical isolator, and then separated by a tunable filter and a second optical circulator, then the frequency converted light is outputted from the port of the circulating frequency shift module. The frequency converter provided in the present invention has a compact volume and fewer optical components, which means it is easy to be integrated, a big frequency converting range, a high frequency converting efficiency and a high output power.