Embodiments of the present invention provide a core network device, an access network device, and a data offloading method and system, and relate to the field of mobile communications. In the system, the access network device receives an offloading inclination indication of service data from the core network device; the access network device allocates, according to the offloading inclination indication, a transmission resource used for transmitting the service data; and the access network device transmits the service data by using the allocated transmission resource. In the present invention, quality of service (QoS) in the transmission process can be ensured even if data is offloaded by using a first resource such as an unlicensed spectrum, a transmission resource provided by Wireless Local Area Networks (WLAN), or a transmission resource provided by the secondary eNodeB.