Actively mode-locked fiber lasers based on the semiconductor optical amplifier(SOA),which are capable of generating ultra-short pulse train with high quality and high repetition rates,received considerable attention. After considering the nonlinear polarization rotation in the SOA,a theoretical model of the harmonic mode-locked fiber ring laser based on SOA is established. Using this model,for different bias currents of the SOA,the influences of the nonlinear polarization rotation in the SOA on the shape,peak power,and pulse width of the pulse output from the harmonic mode-locked fiber ring laser are investigated theoretically. Numerical results show that the mode-locked pulses whose peak power and pulse width are 0.16 mW and 6 ps for the low currents,0.56 mW and 19.8 ps for the high currents respectively,can be obtained for the harmonic mode-locked fiber ring laser.