Enable Developer Options on MIUI 8 & MIUI 9:
Step 1: Open Settings app on your Mi device.
Step 2: Scroll down and tap on About Phone.
Step 3: Tap on MIUI version for about 7-10 times.
Step 4: You should see a message saying “You are now a developer”. That’s it now Developer Options are enabled on your device.
Enable USB Debugging on MIUI 8 & MIUI 9:
Step 1: After Enabling Developer Options, open Settings app again.
Step 2: Go to Additional Settings > Developer Options.
Step 3: Look for USB Debugging under DEBUGGING section.
Step 4: Turn on the toggle beside USB Debugging and tap on OK.
Congratulations! Now you have successfully enabled USB Debugging on your Mi device running MIUI 8 or MIUI 9.