Xiaomi have locked bootloader for for all of the devices in order to protect user data safety i.e to protect users to getting the counterfitted devices or ROM.
Changes that occur after locking bootloader:
The unlocking procedure will need Internet access to get the unlocking password. Also, the Mi Account logged in on the Mi phone and the unlocking tool needs to be the same. Otherwise the unlocking request will be denied. This will ensure that ill-intentioned people will not get access to your personal data.
Risks of unlocking Mi Devices?
Important:The risks stated above do not fully cover changes which may be brought about by Mi Unlock.We will keep improving Mi Unlock in order to prevent the cases when unauthorised vendors unlock Mi devices to install third party apps which worsen MIUI user experience and cannot to be deleted. Locked devices also provide you with high-quality security features, such as Find device, and other added-value services. We're sorry for any inconveniences our policies may cause. Please think twice before unlocking if you are not familiar with ROM flashing.
Important: State Your Unlocking Reason - Please provide the real reasons why you want to unlock your phone in English only. Copied reasons from other tutorials might cause your unlock application to be rejected .
You must be on Developer ROM to unlock your device. If you're on MIUI Global Stable then please download MIUI Global Developer ROM.
Note: After unlocking bootloader, you will be able to flash fastboot ROMs and custom ROMs. The unlocked device will be marked in the official service end. You could lock bootloader again by flashing official MIUI ROMs. (This will need to use fastboot oem lock. Please note to flash offcial MIUI ROMs, otherwise your system won't be able to start)
Known Issues faced by users :
Stuck at 50%
this is the most common error faced by users, that while unlocking the process stops at 50% giving the error that your account at the device and Mi unlock tool is different.
Solution :if you are getting the same error then we request you to wait for 10 days more as your account details for unlocking might not be updated at the server end. Try after 10 days if still you are getting the error you can post a thread mentioning the following details
USB device problem
For those who received permission to unlock but still stuck, please check out this section. You might be facing problem due to USB problem especially for those who are using Windows 10 which causes your PC fail to detect device in fastboot mode. Download Mi Flash tool.
you will see that driver is reinstalling and show Android Device as Android Bootloader Interface, once the driver finished updating it will revert back to Kedacom. Then perform fllowing
Since the driver is all right now, you can open Mi Unlock Tool and you should be able to unlock without problem now
Mi Unlock Tool:
you can download the Mi Unlock Tool From below
MI Unlock Tool Latest- ---updated/New Flash Tool
[Other] Bootloader Unlocking Tutorial & Unlocking Tool (Updated - 11th October 2017)