A fiber-optic ring laser based on semiconductor optical amplifier (SOA) is proposed. In a certain range, a single frequency output of the laser can be realized. At the same time, a new pulse instability phenomenon is found in the laser. This instability phenomenon and current regulation direction is related. The analysis and comparison experiments show that this phenomenon is caused by the photothermal effect of phase-shifted fiber gratings, and weakening the photothermal effect can make the laser output more stable.