Embodiments of the present invention provide a data transmission method, device, and system. A terminal device determines whether a data volume of uplink data to be sent is less than a preset data volume threshold. If the data volume of the uplink data to be sent is less than the preset data volume threshold, the terminal device establishes an RRC connection to a base station and sends an RRC connection completion message that carries the uplink data to the base station, so that the base station sends the uplink data to an MME by using a signaling message between the base station and the MME. The MME further sends the uplink data to a corresponding application server by using a signaling message between the MME and a P-CSCF.