Xiaomi Mi 5 comes with a locked bootloader. Locked bootloader Mi 5 users can’t flash MIUI fastboot ROM.
This article will demonstrate how to unlock Mi 5 bootloader. The steps describe in the video is clearly explained.
Here is the video showing how to unlock Xiaomi Mi 5 bootloader
As you already know MIUI 8 China Alpha ROM for Mi 5 is available to download now.
The latest version of MIUI 7 Global Stable (V188.8.131.52.MAAMIDD) is also available to download mi5.
However, as MIUI 8 China Alpha ROM doesn’t provide any built in Google apps including Play Store, Gmail, and Map etc. You can install all Google apps using Google Apps Installer app.