Embodiments of the present invention provide a data transmission method and device, which are to be applied to a source client, wherein the source client is a client in a foreground running status in a mobile terminal, the mobile terminal, by means of a split-screen function, divides the display screen thereof into a first split screen for displaying a running interface of the source client and a second split screen for displaying a running interface of a target client, the target client is a client in a foreground running status in the mobile terminal. The method comprises: establishing a communication connection between the source client and the target client; receiving a drag instruction for a thumbnail of target data in the source client and moving the thumbnail according to the drag instruction; monitoring whether a data transmission instruction for the target data is received, if yes, transmitting the target data to the target client through the established communication connection. By applying the embodiments of the present invention, users can make full advantage of the split-screen technology, which simplifies data transmission operation.