HI,
Can you please clear your question?
>By the way, I'm in SPD 2010
What do you mean by SPD 2010. Are you using sharepoint designer to get the value?
>But it can't get user Name automatically like "created by"/"modified by"......
Created By and Modified By columns are default column in every list or library so they get the value automatically when you do some operation in list or library.
Now what you want to do with your custom column type people or group?
Cheers, Hemendra-MCTS "Yesterday is just a memory,Tomorrow we may never see"