A method and apparatus for transmitting messages between heterogeneous networks are disclosed. The method includes: receiving an IP multimedia subsystem (IMS) message from an IMS network; converting the format of the IMS message into the format of a non-IMS message and converting the sender terminal ID of the IMS message into the sender terminal ID of the non-IMS message; and sending the non-IMS message that includes the content and sender terminal ID of the non-IMS message to a receiver terminal of the non-IMS message through the home network of the non-IMS message. The disclosure can implement message interworking between an IMS user terminal and a non-IMS user terminal.