Why does the mobile phone automatically turn off after texting or surfing the Internet? Is it the battery or the phone?

There may be something wrong with the battery and mobile phone.

Check whether the battery has poor contact with the machine, poor contact, etc. It is also possible that the battery is broken. You can try another battery to see if it is a battery problem. If it's not a battery problem, it's just the phone itself. Suggest sending it to customer service for testing!