The present invention discloses an IMS system based session key negotiating method. A symmetric encryption algorithm and a secret key are configured in a multi-media server at a terminal; a calling party uses a first symmetric encryption algorithm and the secret key to encrypt the session key to generate a first ciphertext session key, and the first ciphertext session key is transmitted to the multi-media server as an SIP message via a CSCF server; and the multi-media server receives the SIP message, uses the first symmetric encryption algorithm and the secret key to decode the first ciphertext session key to obtain the session key, the multi-media server uses a second symmetric encryption algorithm and a secret key configured by a called party to encrypt the session key to generate a second ciphertext session key, the second ciphertext session key is transmitted to the called party as the SIP message via the CSCF server, and the called party receives the SIP message, and uses the second symmetric encryption algorithm and the secret key to decode the second ciphertext session key to obtain the session key. The method of the present invention can increase security for transmitting the session key.