  This paper studies the propagation of intense laser in plasmas in weak
relativistic region($0.1<a_0^2<0.5$) using the quasi-periodic approximation.
Effective Lorentz factor and density wave effects are calculated in detail. The
relativistic correction on stimulated Brillouin instability is investigated in
the rest parts. The coupled dispersion relations of Stimulated Brillouin
Scattering(SBS) are obtained and investigated numerically.
