My phone started by automatically switchoff after rebooting

@esnemesis try and start your phone in safe mode. If it stays on in safe mode, you know it it's a bad 3rd party App that causes this. Delete the Apps one by one until you know which one caused. If that did not work, replace the battery. That is to rule out a power issue. Since iFixit does not yet have a guide for this, use this one

If none of that works, you may have issues with your ROM file (firmware) and it may require to be reflashed. Of course you can always try a factory reset, but that will not work if your phone continues to bootloop. To get your phone into safe mode you :

1. Reboot the phone by pressing and holding the power button
2. Wait for the MIUI logo to appear
3. Press and hold the volume down button
4. The word "safe mode" will appear in the lower left corner of the screen
5. Use the volume keys to navigate to the option and reboot your phone in safe mode