We propose a high-Q photonic-electronic hybrid cavity for single-longitudinal-mode narrow-linewidth oscillation, where part of the cavity is in the radio frequency(RF) domain by a pair of frequency conversions. In the RF part, we can easily achieve MHz filtering and a large delay by inserting an electronic filter. In mathematics, we prove that the frequency conversion pair and electronic filter in between can be equivalent to a high-Q optical filter cascaded low-noise optical amplifier as a whole. Finally, the 20-d B bandwidth of oscillation is 1/20 of that of an optical local oscillator, and the maximum phase noise suppression can reach 65 dB. We propose a high-Q photonic-electronic hybrid cavity for single-longitudinal-mode narrow-linewidth oscillation, where part of the cavity is in the radio frequency (RF) domain by a pair of frequency conversions. In the RF part, we can easily achieve MHz filtering and a large delay by inserting an electronic filter. In mathematics, we prove that the frequency conversion pair and electronic filter in between can be equivalent to a high-Q optical filter cascaded low-noise optical amplifier as a whole. Finally , the 20-d B bandwidth of oscillation is 1/20 of that of an optical local oscillator, and the maximum phase noise suppression can reach 65 dB.