Gain fluctuation in a microwave photonic phase shifter based on Brillouin slow and fast light is theoretically and experimentally analyzed. The results show that, in order to reduce the gain fluctuation, it is necessary to increase the vestige sideband suppression ratio of the single-sideband modulation. Additionally, the input power of the optical single-sideband modulation signal should be set at a rational level to maintain a high signal-to-noise ratio and simultaneously keep a low gain fluctuation.