A data transmission method provided in embodiments of the present application includes: determining, by a mobility management entity MME, that user equipment UE changes from an unreachable state to a reachable state; sending, by the MME, a request message to a serving gateway SGW; receiving, by the MME, downlink data sent by the SGW; and sending, by the MME, the downlink data to the UE. In this way, when the UE changes to a reachable state for downlink data, the MME requests the SGW to send the downlink data, and the MME then sends the downlink data to the UE, thereby avoiding loss of the downlink data caused by delivery of the downlink data to the UE that is in an unreachable state, and improving real-time accuracy of transmission.