The stability of reflectivity of stimulated Brillouin scattering（SBS） for KrF laser, which is caused by the fluctuation of pump power density, depends on experimental parameters including the pump energy,pressure of the medium and focal length. The stable SBS output can be obtained when the following conditions are satisfied: the pump energy exceeds 5 times the threshold energy, the SF6 pressure is 16×105Pa and the focal length is within 15—50cm. It is also disc overed that the stability of SBS reflectivity is increased with the SBS reflecti vity. The stability of SBS reflectivity is in fact determined by GIL. The th eoretical calculation results are consistent with the experimental results.