Abstract: The theory and basic properties on the passive phasing in the ring-cavity structure was illustrated. The influence of the focal length on the mode-locking speed was analyzed. An analysis model was established consisting of a broad spectrum amplification model and a simplified model for stimulated Brillouin scattering effect. Speed and stability of the mode-locking in this scheme was shown to be determined by the variance of optical path difference, the dynamic phase noise and the stimulated Brillouin scattering effect. The simulation results show that, using a proper focal length, a fast mode-locking speed is obtained. The phase noise degenerates of the beam quality, and even causes the mode hoping. The stimulated Brillouin scattering effect of the fiber with high power output limits the peak power of each frequency, increases the peak number in the output spectrum, and causes degeneration of beam quality. The mechanism of the mode width decrease with the growth of feedback times is demonstrated in theory. The results show that the stimulated Brillouin scattering effect provides a mechanism of mode width growth, which makes the mode width maintain at a balanced value.