Hi Lu jian,
If you want to import source code in order to review or modify it, you need to run EZ-USB suite(ezUsbSuite) which locates at the install path ~/FX3_SDK_Linux_v1.3.3/ eclipse. After that you could import the source code.
The cyusb_linux tool is what we provide to the customer to talk with the USB devices attached to the host. It works with img file.
So the problem is that you choose the wrong tool.
Thanks you for your help,I found there exist a tool to convert a .elf to .img.