A method for transmitting deferred media information in an Internet Protocol (IP) multimedia subsystem (IMS) includes: a sending party of the media information sends a key generation parameter encrypted with a Ka to a mailbox application server of a receiving party of the media information, the mailbox application server stores or saves the encrypted key generation parameters, and sends the encrypted key generation parameters to a key management server (KMS); the KMS generates a media key K and forwards it to the sending party through the mailbox application server of the receiving party; the receiving party obtains the encrypted key generation parameter from the mailbox application server and sends it to the KMS; the KMS generates the K and sends it to the receiving party; the receiving party decrypts the encrypted media information by using the K. A corresponding system is also disclosed. The method and system decrease signaling interaction between the sending party and the KMS, reduce the storage pressure of the KMS; realize the end-to-end secure transmission of the deferred media in the IMS.