Abstract: A theoretic model is made to interpret how the phase of the pump laser beam couples with the intensities of Stokes waves produced by the pump laser beam transimitting in a long path air, This model has clariffied the physical mechanism of how the stimulated Raman scattering (S RS) influences the quality of the transimitted pump laser beam. the results of the theoretic research have shown it is the SRS and the nonhomogeneity of the pump laser beam that make the wave plane of the transimitted pump laser beam distorted, resulting in its quality degradation. This conclusion agrees qualitatively with the experimental results in which the far-field divergency angle of the transimitted pump laser beam θ_p increases with the increment of the Stokes energy conversion ratio η_s. In addition, the experimental results also show that the pulse duration of the transimitted pump laser beam is almost unaffected by SRS when the η_s <5%. but becomes compressed and " Red shift" takes place in the pulse tail of the transimitted pump beam when η_s>5%.