A data transmission device and a command merging method for data transmission are provided. The data transmission device includes a command register and a command merging unit. The command register receives and temporary storages a plurality of original commands, wherein the original commands include a plurality of memory blocks. When the command merging unit judges these memory blocks of the original commands to be a continuous memory block, the command merging unit merges the original commands into a merging command, and transmits the merging command to a peripheral device. Thus, the multiple commands send by the host can be analyzed and merged by the data transmission device to decrease a number of the commands to be proceed by the peripheral device, so as to speed up a command processing time of the peripheral device efficiently.