At present, most scholars know the importance of requirements evolution, but still lack efficient methods to guide. According to the problems mentioned above, this paper proposes an approach which supports the OWLS requirements specification evolution. By using the meta information of OWLS requirements specification，we can finish the OWLS requirements specification evolution. We propose a reflective requirements specification model based on πcalculus. For the OWLS characteristics, the OWLS requirements specification description method based on πcalculus is given. Using the processpassing mechanism in highorder πcalculus, the evolvement process of requirements specification can be specified. A reasoning and verification method is given for the correctness of reflective requirements specification evolution. At the same time, we use the  traffic traveling examples to demonstrate this argumentation.