virtualbox.org

I mount my mobile disk at USB device list,and my ISO image is on mobile disk.
then the machine auto enter UEFI Shell every time when I reboot until I unmount my mobile disk from USB device list.

Why not just eject the ISO from the optical drive? That is what you would do after you installed an OS on a physical PC - remove the DVD from the drive.

You can do that from the Devices tab if the vm is running, or from Settings of the vm it it is shutdown.