The present invention relates to a data transmission method applied to a data transmission system comprising a data transmission device, at least one host computer and at least one associated computer, wherein all the at least one host computer and the at least one associated computer are disposed with a plurality of switching system clipboards, and have one computer clipboard thereof. Therefore, by the disposing of the plurality of switching system clipboards, two or more users are allowed to simultaneously carry out their computer switching operations and data transmissions between multi computers through one identical data transmission system, moreover, the file size of the transmitted data does not be limited by the data transmission device of the data transmission system.