Low impact development (LID) can effectively repair the traditional rainwater excretion and destruction of the transmission system. In this paper, the SWMM model was established in Yaolingjiayuan district of Tonglu county, and the rainwater process under different rainfall return period was analyzed after the urbanization and the vegetative swale was added. The location of the vegetative swale and the location of the outlet section of the pipeline were compared with the effect of the vegetative swale, and the flood mitigation effect of the vegetative swale was studied. The results showed that the mitigation effect of vegetative swale is different under different rainfall return period. The mitigation effect of vegetative swale is more obvious in low return period and decreases with the increase of the return period. When return period of rainfall increases to a certain value, the mitigation effect will lose. By changing the setting position of vegetative swale, concluded that when vegetative swale is close to the outlet section, the mitigation effect will be more obvious, and with the increase of the distance, the mitigation effect will decrease.