The SIP protocol is a controlling protocol of the application layer. In order to improve the security of SIP protocol, based on the intractability of the elliptic curve discrete logarithm problem, combining with the user's identity, password and one-way trapdoor function F(), this paper proposes an SIP authentication key agreement protocol with the basis of ECDLP. The agreement process consists of four parts:initialization, registration, login authentication, and changing password. The safety analysis shows that the proposed protocol not only provides two-way authentication and a safe session key, but also resists the password-guessing attack, man-in-the-middle attack, replay attack, masquerade attack, and Denning-Sacco attack. Compared with other protocols, the proposed SIP authentication key agreement based on ECDLP has higher security and can better meet the application demands.