A dark pulse optical fiber laser with tunable pulse widths based on semiconductor optical amplifier (SOA) is demonstrated. By injecting a portion of light to the SOA from the output of the optical fiber laser, due to the nonlinear polarization rotation of the SOA, the output pulse experience different polarization evolutions and dark pulse is obtained. By changing the length of the feedback cavity, dark pulses with different widths are obtained. Nonlinear polarization rotation effect of the SOA and the mechanism of the dark pulse production are analyzed. In the experiment, the output of the dark pulse with tunable widths is observed. By changing the length of the feedback cavity, under the condition of fixed repetition frequency(11.31 MHz), dark pulses with tunable widths of 5.91~22.34 ns are obtained.