I set default site's URL in edit default site page, add an URL localhost/cms in URL list, when click save button, there is an error "Invalid site URL: 'localhost/cms'.", how to work around this error or are there any other way?
Currently this is not possible, mainly because of the virtualization technique sensenet uses. It basically takes over the url and serves everything from the db. Maybe there is some kind of url rewriting trick that operators can do, but I don't know of a working example.
So you have to deploy sensenet to an application root, it cannot live in a virtual directory or subfolder.