This study proposes a method for suppressing phase noise of an optoelectronic oscillator (OEO) based on self-gain modulation effect of a gain-saturated semiconductor optical amplifier (SOA). The SOA is added into the OEO loop and operates in gain-saturated state by adjusting the optical attenuator and drive currents. Partial amplitude noise in modulated optical signal is suppressed due to the SOA's self-gain modulation effect, thereby improving the signal-to-noise ratio of the loop and reducing the phase noise of generated microwave signal. Furthermore, the composition of phase noise in the OEO and the noise-suppression principle of the SOA are analyzed and experimentally verified. The experimental results indicate that when we add a gain-saturated SOA in a 5-km-long single-loop OEO, the near-end phase noises of 10 GHz microwave signal are -84.5 dBc/Hz and -113.9 dBc/Hz under 0.1 kHz and 1 kHz, respectively, i.e., an improvement of 3 dB over that without the SOA. The experimental results are in agreement with the theoretical analysis.