U-boot is an open source project under the protocol of GPL, which has been widely used as boot application on embedded system. Normally, U-boot initializes hardware and send to hardware information to the embedded OS. In mobile thermals, especially in the arm-based mobile device, U-boot has played an important role. U-boot supports many kinds of development boards. However it does not support every board, so it needs to be modified to fit other board. This paper analyses the boot procedure and the source code of U-boot, studies how to add a custom command on U-boot source, analyses the protocol of USB, then gives us a new way to download the binary U-boot file or the kernel directly to RAM and run. In this way, we do not need to burn the binary U-boot file or the kernel to Flash. This has save a lot of time in embedded system development.