A controlling method and device for data transmission including the steps of providing a system bus for connecting a first transmission channel and a second transmission channel with a command processor, adjusting a transmitting direction of the system bus according to a transmitting direction of the second transmission channel, and proceeding the data processing procedures of the second transmission channel, wherein parts of data processing procedures of the first transmission channel will last during a interval between the system bus adjusting the transmitting direction and the data processing procedures of the second transmission channel start on.The present invention ensure that the independence between every data caching and processing reduces the times of flushing the cached data from the data transmission channel and re-seeking through the source, shortening the transmission time, increasing facileness and improving the efficiency of the data transmission.