How to access App Store using only IPv6?
Just post my settings in case anyone need it.


System Preferences -> Network -> Advance


  TCP/IP: set "Configure IPv4" off. 
  
  DNS: add 2001:778::37, and drag it to the top


Now you should be able to see contents in Mac app store. And it might take quite a while to establish connections for downloading. Just be patient.
I also found a line in my /etc/hosts (copied from this IPv6 hosts project, which was up-to-date and very useful for IPv6 users in mainland China):


  2600:140b:4:18c::c77 www.apple.com #e3191.dscc.akamaiedge.net


I'm not sure if it affects the result. If Step 1 doesn't work, you can consider to add it into your /etc/hosts file. Just remember to make a backup first.