How to pair Redmi Watch 2 Lite with Android phone?
Redmi Watch 2 Lite needs to be bound by Mi Fitness App. After binding, lock the app in the background to avoid being shut down by the system and disconnected from the phone, which will affect the normal use of the watch.
1) Press and hold the power button for about 3 seconds to turn on the watch. Scan the QR code on the watch screen or search "Mi Fitness" in the application market through mobile phone to download and install Mi Fitness APP.
2）Open the Mi Fitness app and follow the pop-up box to open the app-related permissions. Click "Add device", click Auto Scan Device or Redmi Watch 2 Lite. 
3) When the Bluetooth name of the watch appears, click to pair it. 
https://www.mi.com/global/support/faq/details/KA-07273/