Abstract: The output characteristics of a passively mode-locked erbium-doped ring fiber laser with a semiconductor saturable absorber mirror were studied. This ring fiber laser was constructed with a 1480nm laser diode as pump source, erbium-doped fiber as gain medium, as well as circular, polarization controller, wavelength division multiplexier and coupler. In experiments, the fiber laser operated stably with mode-locked pulse laser output, whose peak wavelength of 1586nm, spectrum width of 4.8nm, repetition frequency of 11.2MHz and maximum mean optical power of 8.4mW. It was found that the characteristics of the output laser pulse in time domain would change a little while adjusting the polarization controller in experiments. Hence, the effect of the polarization change in the fiber laser on characteristics of the output laser pulse should be paid attention to in practical utilization. The experimental results are helpful to the use of semiconductor saturable absorber mirror in passively mode-locked fiber lasers.