this is resolved.
just for other's info:
as there are 1000s of accounts on server, private cache can not be used because it is inconvenient to disable private cache (via rewrite rule in .htaccess) for others. In this case, only public cache should be used. To enable public cache for such an account, do this:
1. Cache Policy in Admin CP
Enable Cache: No
Enable Private Cache: No
2. Add rule in very top of .htaccess for the account to enable cache
RewriteRule ^(.*)$ - [E=Cache-Control:max-age=120,L]