We have one file server at local office and we want to setup another standby file server at remote data center, the two side will be connected with VPN, instead of enabled dfs on both servers and let the file(200GB) at local office to slowly replicate to the remote server, can I first backup the data on local server to like external HDD, restore to remote server then enable DFS replication? So that only file change will be replicate ?
The term for what you're asking is called preseeding, and yes it is possible.
You can do it many different ways, such as using robocopy or Windows Server Backup.
To use robocopy, an example command would be:
robocopy "<source replicated folder path>" "<destination replicated folder path>" /e /b /copyall /r:6 /w:5 /MT:64 /xd DfsrPrivate /tee /log:<log file path> /v
This is taken from an article by Microsoft on the topic here: https://technet.microsoft.com/en-au/library/dn495052.aspx