When my phone is hot its automatic power off

This is a feature not a bug...

It keeps your phone from dying from over-heating... so the questions then become Why is the phone overheating? and How can you keep it as cool as possible?

Okay, start by turning on any power-saving features the phone has to reduce power demand where possible. This would imply turning off things you aren't using like bluetooth , wifi, NFC.

Reduce the screen brightness to no more than you need!

Getting back to the first question though the answers are:

1. Making it work too hard. To avoid this try to limit the number of apps you have open at once. If you game on your device with the latest greatest games they tend to require a lot of work from your phone, if you use video heavily as in video calling, video editing or streaming video on-line these all require the phone to work harder, solution: Do that less, or stop when your phone gets warm, and before it shuts down.

2. External factors such as the ambient temperature, if you are in a hot environment the phone will obviously overheat quickly, just like you do. About the only thing you can do for that is don't make it work as hard when it's too hot, just as you avoid heavy labor in extreme heat... and keep it out of the direct sun, just like you do when you want to avoid heat stroke... and just like you take off clothes when it's too hot, you can take the case off your phone when IT is too hot, that lets the air get to it and carry away heat better.

In theory, keeping your software up-to date also helps with keeping it cool (I have my doubts) but software certainly CAN be a factor, so only put the stuff you need that works well and that doesn't cause problems, if it does or isn't needed, get rid of it. Sadly phones come with LOTS of stuff that falls into these categories but which you CANNOT remove. Again in theory outdated software can be a cause... consider that but be skeptical too. my experience says, if the old stuff still works, it will OFTEN be better than the new improved version. DO keep an eye out for apps that seem to cause problems, there are poorly designed and malicious apps in the world... "where did you get them?" should be a question that is helpful, and IS to a degree, but the venerable Company-who-shall-not-be-named's AppStore is, sadly RIFE with malicious and poorly designed software.

Last, we can look into possible hardware issues. Problems with chargers, cables and port and part failures... use the adapter and cable that came with your phone... Use Fast Charging only if you need to, it generates more heat and shortens battery life.

If you implement the changes listed you may find overheating to be less of a problem, if not, you might report back to us and see if someone else has better or more advice for you, but you can start there.

Good Luck.

P.S. heavy use while charging tends to also be problematic and a poor idea...