Many many thanks to Shawn Feldman!
Quote from him:
"I was having a few issues getting smart search installed and granting it the right permissions to access the db (I’m running a multi account environment) so I put it in a feature, Gac’d it, and added SPSecurity.RunWithElevated for whenever it makes a database connection. Here’s the code if you’re interested."
It's my fault. I know this problem when I was working on a Internet(anonymous) facing SharePoint site. The problem of SmartSearch v1.1 is it will use the current user to access db, of course, cannot be successful when using IUSR_XXXX accounts. RunWithElevated is the definitely the solution but I don't have time to do as I'm alwasys travelling around.
So here you are:
I think this is the reason why we made these open source tools - everybody can contribute to it, and make our work better.