What happens if you force a power down by pressing and holding Power for about 10 seconds until the phone shuts down. Does it try to turn on again afterwards? If so, there might be a problem with the Power button's circuits.
If it's a problem with the power button's circuits, you'd probably have to bring the phone to a repair shop or a Huawei service center. It's not the kind of thing I'd try fixing on my own.