A method for key management and assignment in MBMS service, the method includes following steps: the group key locates in the root node on the highest layer, which only has child nodes and doesn't have parent nodes; private keys corresponding to users locate in leaf nodes; the described intermediate node that owns both one parent node and one or more child nodes holds it own key. This invention deploys the method of combining point-to-point mode and point-to-multipoint mode during the process of key update; compared with the key update method only deploying point-to-point mode, this method can reduce the times necessary for information transmission, reduce the system load as well as the time needed for one key update process. Compared with the key update method only deploying point-to-multipoint mode, this solves the security problem of key exposure.