An actively mode-locked fiber ring laser based on cross-gain modulation (XGM) effect of semiconductor optical amplifier (SOA), with a small intracavity loss, is proposed and demonstrated. By using an optical circulator, the intracavity loss was reduced and the output power of the laser was increased. The physical mechanism of pulse multiplication in ring cavity during rational harmonic mode locking process has also been theoretically analyzed in detail. Taking advantage of rational harmonic mode locking technology, an optical pulse train with a repetition frequency of 30 GHz was obtained using a modulation frequency of 10 GHz, and the peak power was about 0.5 mW. With broad gain spectral width of SOA and large tuning range of filter, the output power of the output pulse train could keep big enough over a large tuning range. Harmonic mode locking at 20 GHz was successfully achieved and the peak power of the output pulse train was more than 0.65 mW over a 40 nm tuning range. The semiconductor optical amplifier and the short length of the ring cavity ensure the long-term stability of the laser.