Abstract: Optoelectronic oscillator (OEO) is a novel promising high quality microwave signal generator. The OEO′s high frequency, multi-mode oscillation character requires that there must be a narrow band filter in the loop. The method of reducing phase noise by increasing the fiber length is limited by the filter′s relative bandwidth. An assistant frequency conversion technology is innovated.It can be broaden the filter′s relative bandwidth by lowering the filtering frequency, and cancel the local oscillator′s phase noise at the same time. The OEO with this technology can break through the fiber length limit theoretically, and has lower phase noise. Experiments are done to compare the phase noise of OEOs with and without the technology. It is clear that the OEO with this technology can use the longer fiber and reduce the phase noise significantly.