In this paper, a single mode optoelectronic oscillator assisted by active ring resonance cavity filtering is presented and verified. Using the high Q optical comb frequency response to select the oscillation mode of an optoelectronic oscillator, the system can effectively suppress the side-mode and generate single mode signal. Theoretically, the optoelectronic oscillator oscillation mode and the frequency response of the active cavity are analyzed. The simulation results show that the active ring resonance cavity filtering is of benefit to the side-mode suppression and single mode output in an optoelectronic oscillator system. By comparing with experimental result, the theoretical prediction is verified. The output of a 20 GHz single-mode signal with a side-mode suppression ratio of 58.83 dB and a phase noise of -97 dBc/Hz at 10 kHz from carrier is also obtained. This scheme has the advantages of the existing optoelectronic oscillator side-mode suppression methods. In addition, it has more convenient manipulation, and good flexibility and tunability.