The reason why Xiaomi's mobile phone automatically shuts down and enters the main menu is that the system is out of order or unstable.

1. This situation is usually caused by system abnormality, system upgrade or software conflict.

2. There are also reasons caused by mobile phone hardware or battery problems. You can try to restore the phone to the factory settings to solve the problem, which will erase all data and all internal storage.

3. If the problem still exists, you need to check the hardware and battery status of the mobile phone, or seek the help of Xiaomi mobile phone customer service.