The invention discloses a data transmission system and method. The method is applied to a mobile terminal and an external device which is connected with the mobile terminal through a preset interface. The mobile terminal comprises a first modulator-demodulator connected with a first subscriber identity module and a second subscriber identity module; and a first application processor. The external device comprises a second application processor and a second modulator-demodulator. When the second application processor entering a wakeup state from a sleep state through a handshake keyword detects a data sending instruction, the second application processor detects whether a data packet in a sending state exists in the preset interface or not; and if the data packet in the sending state does not exist in the preset interface, after first preset time, the second application processor sends a to-be-sent data packet to the first application processor in the wakeup state through the preset interface. According to the system and method, a double-LTE communication process is realized by the mobile terminal through the external device, and the accuracy of identifying the data packet by the mobile terminal is improved, wherein the data packet is sent by the external device.