When stimulated Brillouin scattering (SBS) is applied in laser drive system, it is necessary to consider the changes of SBS pulse shape, especially the leading edge shape, compared to pump pulse. For the pump pulse with several ns pulse width and 2ns leading edge, adding a small slow rising signal or a pre pulse at the root of pump pulse steep leading edge, then the SBS pulse shape is investigated for different two cell SBS phase conjugation (PC) mirror. The results show that the SBS leading edge shape can keep almost the same as the pump pulse for short amplifying cells and short focal length. The intensity ratio of SBS main-and pre-pulse is changed with different focal lengths for short amplifying cell.