Before proceeding to unlock bootloader on Xiaomi or Mi Phone make sure to backup all of the data to a safe place, as unlocking bootloader may wipe all of your data and settings.
How to unlock Bootloader on Xiaomi or Mi Phone using Mi Unlock Tool:
Step 2: Go to http://en.miui.com/unlock/ and apply for unlock permissions by filling up the details explaining why you want to unlock the device’s bootloader.
Step 3: Once the application is submitted it will be manually reviewed by the Mi Team. If approved you will receive an SMS stating that your application is now approved. The reviewing process may take up to 3-21 days.
Step 4: After the approval, download Mi Unlock Tool (Link) and run it on PC as administrator.
Step 5: Login with your Mi Account and follow the on-screen instructions.
Step 6: When asked to connect your device to PC, connect it using the appropriate OEM USB Cable.
Step 7: Once the device is detected the unlock process should start automatically. If the device is not detected then download and install Xiaomi USB Drivers from here.
This is the complete guide on how to unlock bootloader on Mi or Xiaomi Phones using Mi Unlock Tool. Have any questions, then ask them in the comment section below.