In this paper, the generation of dual-wavelength stable nanosecond pulses by a laser diode pumped Yb-doped double-clad fiber laser is presented. In the experiment, the fiber laser with two-mirror cavity is approved which operates in a self-Q-switching regime. The Q-switching mechanism is based on stimulated Brillouin scattering (SBS). When the pump power achieves the SBS threshold, the fiber laser changes from the start resonator to the SBS resonator. With different reflectivities of the second mirror, stable dual-wavelength pulses with the pulse width range from 10 ns to less than 2 ns are obtained. The result was explained theoretically by birefringency (including stochastic birefringency and bend birefringency).