The electrostrictive force induced by beam interference can drive the acoustic wave,and enhance the stabillty of stimulated Brillouin scattering (SBS) phase conjugation fidelity. For the reflecting cell configuration pumped with a 2ns steep leading edge laser pulse, taking a two stage Nd glass amplifier and a SBS cell as a whole,it is shown that the measured far field distribution and phase conjugation fidelity of the output laser beam are more stable for θ≌5 mrad than those for θ>30 mrad.