A message transmission method and a core network interface device are provided, so as to reduce complexity of a communication interaction between a core network and each external entity. After receiving a request message from a service instance, obtains device indication information of a target device, an identifier of the service instance, and a type of a response message corresponding to the request message; after sending the request message and receiving a to-be-forwarded response message, obtains a type of the to-be-forwarded response message and device indication information of a device that sends the to-be-forwarded response message; sends the to-be-forwarded response message to the service instance when the type of the to-be-forwarded response message and the device indication information of the device that sends the to-be-forwarded response message respectively match the type of the response message and the device indication information of the target device that correspond to the request message.