In the communications system, a user equipment UE accesses a core network via a first network-side device by using a first air interface and connects to the first network-side device via a second network-side device by using a second air interface to access the core network. The method includes: acquiring, by the network-side device, an input parameter; calculating, by the network-side device, an access stratum root key KeNB* according to the input parameter and an access stratum root key KeNB on the first air interface, or using, by the network-side device, the KeNB as the KeNB*; and generating, by the second network-side device, an access stratum key on the second air interface according to the KeNB*, or sending, by the first network-side device, the KeNB* to the second network-side device.