Our exchange server system is 2010 sp1. The user mailbox is on the new DB in our DAG But his mailbox showed Disconnected Mailboxes on the old DB. What is the matter of it?

So the mailboxes were moved to this new database? That is expected then in SP1: http://technet.microsoft.com/en-us/library/bb232039.aspx When mailboxes are moved from a Microsoft Exchange Server 2010 Service Pack 1 (SP1) database to any other database, Exchange doesn't fully delete the mailbox from the source database upon completion of the move. Instead, the mailbox in the source mailbox database is switched to a soft-deleted state. With soft-deleted mailboxes, you can use the MailboxRestoreRequest cmdlet set to access mailbox data during a mailbox restore operation. Soft-deleted mailboxes are retained in the source database until either the deleted mailbox retention period expires or until the Remove-StoreMailbox cmdlet is used to purge the mailbox.

Thanks for your great support