The generation of nanosecond-scale pulsed laser with stimulated-Brillouin-scattering (SBS) phase conjugation in fiber was investigated theoretically and experimentally. Numerical simulation was performed to study the waveform and the sequence of laser pulse. The obtained waveform is coincident with the curve describing the relationship between reflectivity of SBS phase conjugation mirror and time. The results show that changing Q value in resonant cavity by utilizing SBS phase conjugation is feasible. To investigate the output laser at different lengths of SBS medium, an all-fiber Er3+-doped Q-switched laser was designed. When the length of single-mode-fiber (SMF) is 1.5 m, laser pulse are generated with pulse duration of 2.6 ns, pulse period of 58.23 ns, average output power of 7.35 mW at pump power of 45 mW. When the length of SMF is decreased to 1 m, pulsed laser with remarkable dual-peaks is obtained with pulse period of 26.47 ns. While the length of SMF is increased to 2 m, only irregular stochastic pulsations are observed. Experimental data indicate that the length of SBS medium is vital to the formation of phase conjugation mirror. If it is too long, pulsed laser can not be generated for the lack of fixed conjugation relation between Stokes lines, and on the contrary, dual-peaks laser pulses occur for the change of photon lifetime in orthogonally polarized modes. Once the laser pulses are generated, their characteristics are only correlated with the stochastic characteristics of SBS and the pulse profile maintains constant with the increase of pump power.