Abstract: A feasible solution is proposed to the security problem of SIP protocol in softswitch since no security strategy has been defined by SIP itself. The function characters and working mechanism of SIP protocol in softswitch, especially its existing security problem are described briefly. Then the security strategy, encryption process, security and performance of PGP (Pretty Good Privacy) which is originally designed for encryption and signature of e-mail are analyzed in detail. The widely-used highly secure PGP security strategy is thus introduced into SIP protocol to meet the security requirements of SIP message transmission. Expatiates the implementation of PGP in SIP protocol is expatiated and the formats of SIP INVITE message with PGP encryption and with digital PGP signature is examplified.