Security protocols play more and more important role with the bursting development of Internet. Formal authentication is an effective way to detect flaws in those protocols. There are many automatic checkers for verifying protocols, but few can deal with digital signature protocols. This paper proposes a new tool for formal automatic verifying. It can have an effective check for digital signature protocols. It is based on intruder's role impersonate and backward deduction. It was implemented by using JAVA.