Stimulated Raman scattering (SRS), which is the basic problem in laser plasma interactions, was studied by a Vlasov-Maxwell code developed by us. It was found that, there is a close relation between the occurrence of SRS and the electron velocity distribution. When a vortex is formed in the phase space of electron velocity distribution, the amplitude of back SRS increases, and when the distribution function is flattened in the phase velocity of plasma waves, the SRS ceases. In our simulation, we clearly observed the pictures of the burst of SRS, the formation and vanishing of the vortex in the phase space of electron distribution function and electron trapping, etc.