Since the first day I tackle this problem, I encounter all kinds of situation.
First way to solve it, is deleting the whole directory of hadoop and then reinstall it.
Then I try to change the namespaceID in /data/current/BP````/current/VERSION and make it the same with that in /name/current/VERSION.
It worked yesterday but it malfunctioned today.
Holy ****.
Then I delete the /name and /data and regenerate them. The problem seems to be fixed.
God knows what kind of new problem will appear tomorrow.