Based on the well known optical them distribution in free space a generalizedstimulated Raman scattering (SRS) coupled wave equation is deduced and a simplemethod to calculate the temporal and spatial distribution of pump beam and Stokes beamin SRS is proposed in the paper. The results obtained by this method show that, for SRSStokes beam, not only the intensity is enhanced but also the pulse duration can becompressed. The principle and conditions being satisfied for beam clean-up are discussed. The merits of SRS in optical communications for compensating both the intensity loss and pulse expanding due to fiber dispersion are emphasized.