VoIP (Voice over IP) is to pay much attention to network application service. The traditional telecommunication was very expensive that caused people pay more attention to VoIP for more cheap and convenient. Because the developing of network is very fast and people pay more and more attention to VoIP, there were some problems of VoIP and waiting for a solving. This thesis wants to discuss the Session Initiation Protocol (SIP) of many protocols in VoIP and try to point out the weaknesses and threatens in the protocols. Our main research is about the authentication and we advanced a new authentication scheme to resist these dangers of security. Then we analyze the security and efficiency of this thesis. Finally, we did our conclusion and our goal in the future.