What is the easiest way from the Beijing airport to Citytel Inn (Block CD in Wangfudjing St) by train?

By train, I think you may mean subway train, right?
If so, you can take airport express to reach Dongzhimen Station where you can change to subway line 2 to reach Jianguomen Station. Upon reaching Jianguomen Station, you can change to line 1 to reach Tianamen East Station. Then you can walk along Nanheyan Avenue to reach your hotel easily.
Or you can take the airport shuttle bus line 3 to reach Chaoyangmen Station and then change to take a taxi to reach your hotel. I think this is much convenient for you.

Thank you for the help!