If you have signed into your Mi account on your mobile phone, please do go to "Settings"-"MI Cloud"and sign out first.
1. Please visit:
https://account.xiaomi.com/pass/del and sign in.
2. Please tick “Yes, I want to permanently delete my Mi Account and all its data”, and then click "Delete Account".
3. After that you need to pass through the verification page: phone number or email.
4. Please continue to click "delete Mi account" and the whole process will be finished.
Please kindly check the attached pictures for detail steps for your reference.