The present application relates to the field of wireless communication technology, in particular to a data transmission method and device, used to solve the problem of there being no solution for sending cached downlink data in a source communication system to a UE after the UE crosses the communication system. An MME entity in a second mobile communication system of an embodiment of the present application receives a downlink data cache indication sent by an SMF entity located in a first mobile communication system; the MME entity initiates the establishment of a user plane connection and/or control plane connection of a UE, so that a user plane functional entity sends the cached downlink data to the UE by means of the user plane connection and/or control plane connection.