The present invention discloses a sub-rate transmission method for user data services in transmission devices of a Metropolitan Area Network (MAN). The method comprises the steps of: pre-configuring each transmission device to form forwarding table items representing corresponding relationship between self-defined tags and transmission lines; after receiving a user data frame, an original transmission device nesting a self-defined tag into the user data frame according to the forwarding table items to form a self-defined data frame, then transmitting the self-defined data frame; and after receiving the self-defined data frame, a terminal transmission device determining an output port for transferring the data frame by searching the forwarding table items with the nested self-defined tag, then deleting the nested self-defined tag and sending the original user data frame to the output port. The present invention, by nesting a self-defined tag, can greatly increase the number of Virtual Local Area Networks (VLANs) accommodated by a MAN as well as simplify the configuration of tags for user data to bring more convenience to users.