First, we need to prepare at least 8GB USB drive and make sure that there is nothing in it that you want to keep, as the following tutorial will erase all the data on the USB drive.
1. Download the beta version of MacOS Sierra on the Apple developer website.
2. After the download is complete, turn off the pop-up installation prompts.
3. Open Finder-Application, right click on "Install 10.12 Developer Preview" and select Display package content.
4. Then open the Content-resource in the pop-up window and locate the Createinstallmedia file.
5. Open a terminal window, enter sudo, press SPACEBAR, and then drag the Createinstallmedia file to the terminal window.
6. Input--volume, then press the space bar, will be connected to the Mac USB drive to the terminal window.
7. Enter the--applicationpath, and then press the space bar, the third step of the "Install 10.12 Developer Preview" to the terminal window, and press the ENTER key.
8. When prompted about whether to continue, enter "Y" and press ENTER.
9. The system will empty the contents of the U disk, and the installation files will be copied in, the entire process will last 20-30 minutes.
After the above operation, we can install the pure version of the MacOS Sierra system via U disk.