1. Go to "Settings" on the phone;
2. Tap "My device"
3. Find "All specs"
4. Tap MIUI version 7-10 times to make Developer options available. (You will see "You are now a developer!")
5. Go back to the previous page and find "Additional settings";
6. Find the "Developer options";
7. Turn on "USB debugging";
8. Tap "OK".
Note: Xiaomi devices need to enable “USB debugging （Security settings）”
PS: You need to sign in your Xiaomi account in settings so you are allowed to enable “USB debugging （Security settings）”
Now you are ready to run non-root setup to remote control your device. (How to run Non-Root setup to remote control Android Phone with AirDroid?)
If you have done all the above steps, but still can't turn on the USB debugging of your device, please kindly submit a ticket.