how can i sign in ftp client(winscp) as root?
Solution 1:
FTP protocol does not allow this. The SFTP and SCP protocols allow for this, but the actual method is platform dependent.
You cannot use sudo from the terminal, so you need to specify it as the "Shell" for the session. This will also require configuration changes in the server.
For the exact details, see the WinSCP FAQ How do I change user after login (e.g. su root)?