An optoelectronic oscillator with assistant filtering by means of the frequency selective characteristic of optical fiber ring is proposed and verified. The dual-loop structure composed of a polarization beam splitter (PBS) and a polarization beam combiner (PBC) is used to suppress the edge of the optoelectronic oscillator. Furthermore, the mode of the optical signal in the oscillator loop is selected again by means of the optical comb frequency characteristic of optical fiber ring, which plays the role of side mode suppression and improves the quality factor of the resonator. The side-mode rejection ratio is over 160dB and the mode interval is up to 370MHz in the case of ensuring low phase noise and high spectral purity of the microwave signal by the optical oscillator, which reduces the performance requirements of the band-pass filter, and is a new design scheme of the optoelectronic oscillator.