iPhone goes off at once

Aziz, you haven't really given us much to go on here.

Is there any particular event that happens when it turns off, or does it seem to be random? Does it turn off when you're using it or when it's idle?

When it goes off, does it seem to be powered off or just that the screen is blank? Does it power back on with no problem? What condition is your battery in; can you check the battery health and add information that to your question?

As you can see, there are a lot of details that may help us figure out what's going on with your phone, so the more you can tell us about it, the better chance we have of being able to help.