My iPhone 15 plus turns on and off when charging

Perhaps you should try a force reset ⇢

Force restart iPhone - Apple Support

If a force reset is without success, I recommend you take your iPhone to your nearest Apple Store or Apple Authorized Service Provider to have it taken a look at. It's possible that internal hardware damage in your iPhone is causing the issue. ⚛️

⇢ Genius Bar Reservation and Apple Support Options - Apple

⇢ Find Locations

If it's the *screen turning on and off it could be that the charging cord is not snug in the port, could have debris blocking a secure connection and causing the phone to wake up. Carefully clean it out and see if that fix's the problem.

Disregard if it's *powering on and off thought. That sounds like a hardware problem.