Q: hi, i am trying to switch off my phone and after a few second it restarts automatically. what can i do?

Try:

- Reset the iOS device. Nothing will be lost

Reset iOS device: Hold down the On/Off button and the Home button at the same time for at

least ten seconds, until the Apple logo appears.

- Reset all settings

Go to Settings > General > Reset and tap Reset All Settings.

All your preferences and settings are reset. Information (such as contacts and calendars) and media (such as songs and videos) aren’t affected.

- Restore from backup

- Restore to factory settings/new iOS device.

- Make an appointment at the Genius Bar of an Apple store.

Apple Retail Store - Genius Bar

How are you trying to turn it off, holding the off button down or both the home and off button down?

Is it plugged in to a charger or your computer?  Unplug it and try the above suggestions.