You can unlock the phone with bluetooth devices such as some wearable devices made by Xiaomi.
You need to set screen lock before you set up Unlock with Bluetooth device.
1. Go to
Settings > 
Passwords & security > Unlock with Bluetooth device.
2. Tap
to enable Unlock device.
3. Verify your identification with a screen unlock.
4. Tap Allow to turn on Bluetooth in the pop-up window.
5. Tap an available device to connect.
You need to set screen lock before you set up Unlock with Bluetooth device.
1. Go to
Settings > Passwords & security > Unlock with Bluetooth device.2. Tap
to enable Unlock device.3. Verify your identification with a screen unlock.
4. Tap Allow to turn on Bluetooth in the pop-up window.
5. Tap an available device to connect.