Can I exit Emulationstation and use the Linux desktop？ Sometimes I want to do simple office work in coreelec。 (E.g. vncview, offices, pdf)
No, CoreELEC is a head less Linux distribution only for Kodi. There does no desktop exist at all.
If you would rather have a Linux desktop computer and still be able to use kodi on your TV Box, then perhaps contemplate to install uBuntu (preferably on a USB memory stick).