@Ligang: please read this blog. From SDK v7.0 the init packet is required. You can't use just the .hex file to do DFU update. You need the init packet and the hex packed in a .zip file.
Good news is you can generate the .zip file using our provided tool. Please tell the SDK version you are using, and please follow the documentation provided in the SDK on how to test the DFU bootloader.
