In order to investigate the mechanism of slow-light effect via stimulated Brillouin scattering (SBS), the occurrence of the anti-Stokes scattering of Stokes light and the relationship between the anti-Stokes scattering and the mechanism of slow-light via SBS were researched. It is proved that the Stokes light can also be scattered by Brillouin ultrasounds besides the pump light in SBS. The scattering is anti-Stokes, and is a reversal process of the Stokes scattering of the pump light. Therefore, the pump and Stokes lights in SBS experience a back-and-forth scatterings in Brillouin sound fields. On this premise, a model was presented to depict the slow-light effect, in which the optical distance of Stokes wave was enhanced due to the back-and-forth scatterings and the pulse was slowed down as a result. Experiments show the deduced delaying time for the Stokes pulse given by the model agrees with former results. Which suggest that the Stokes light can be anti-Stokes scattered in Brillouin scattered by Brillouin sound fields and also that the enlarged optical distance due to multiple scatterings would be the origin of light slowing in SBS.