A deregistration method, comprising: determining, by a terminal device, that the terminal device enters a deregistration state of a first access network, comprising: if the terminal device is in an idle mode for a time period longer than specified duration, determining, by the terminal device, that the terminal device enters the deregistration state of the first access network, or if the terminal device receives second instruction information from a first core network element, determining, by the terminal device according to the second instruction information, that the terminal device enters the deregistration state of the first access network; and setting, by the terminal device, a status of the terminal device to deregistered from the first access network; characterized by receiving, by the terminal device, information about a third packet data unit, PDU, session from a second core network element, wherein the third PDU session is a PDU session that is set up by the terminal device on the first access network and accessible on a second access network and that is determined by the second core network element; and determining, by the terminal device based on the information about the third PDU session, a PDU session accessible on the second access network; wherein the first access network is a 3rd generation partnership project, 3GPP, access network and the second access network is a non-3GPP access network. determining, by a terminal device, that the terminal device enters a deregistration state of a first access network, comprising: if the terminal device is in an idle mode for a time period longer than specified duration, determining, by the terminal device, that the terminal device enters the deregistration state of the first access network, or if the terminal device receives second instruction information from a first core network element, determining, by the terminal device according to the second instruction information, that the terminal device enters the deregistration state of the first access network; and if the terminal device is in an idle mode for a time period longer than specified duration, determining, by the terminal device, that the terminal device enters the deregistration state of the first access network, or if the terminal device receives second instruction information from a first core network element, determining, by the terminal device according to the second instruction information, that the terminal device enters the deregistration state of the first access network; and setting, by the terminal device, a status of the terminal device to deregistered from the first access network; characterized by receiving, by the terminal device, information about a third packet data unit, PDU, session from a second core network element, wherein the third PDU session is a PDU session that is set up by the terminal device on the first access network and accessible on a second access network and that is determined by the second core network element; and determining, by the terminal device based on the information about the third PDU session, a PDU session accessible on the second access network; wherein the first access network is a 3rd generation partnership project, 3GPP, access network and the second access network is a non-3GPP access network.