An inter-network change method, comprising: obtaining (S101), by a terminal, second network slice information to which first network slice information is mapped in a procedure of registering or establishing a session with a first network, wherein the first network slice information is information about a first network slice accessed by the terminal and the first network slice is a network slice in the first network, wherein the second network slice information is information about a second network slice and the second network slice is a network slice in a second network; sending (S102), by the terminal, a request message to the second network, wherein the request message carries the second network slice information, and the request message is used for the terminal to request to register with or be handed over to the second network slice; receiving (S103), by the terminal, an accept message returned by the second network, wherein the accept message carries network slice information accepted by the second network, and the network slice information accepted by the second network comprises a part or all of the second network slice information; wherein the first network and the second network have different access network devices and/or core network devices; and wherein either the first network is fourth generation mobile communications system, 4G network and the second network is a fifth generation mobile communications system, 5G network or the first network is fifth generation mobile communications system, 5G network and the second network is a fourth generation mobile communications system, 4G network. obtaining (S101), by a terminal, second network slice information to which first network slice information is mapped in a procedure of registering or establishing a session with a first network, wherein the first network slice information is information about a first network slice accessed by the terminal and the first network slice is a network slice in the first network, wherein the second network slice information is information about a second network slice and the second network slice is a network slice in a second network; sending (S102), by the terminal, a request message to the second network, wherein the request message carries the second network slice information, and the request message is used for the terminal to request to register with or be handed over to the second network slice; receiving (S103), by the terminal, an accept message returned by the second network, wherein the accept message carries network slice information accepted by the second network, and the network slice information accepted by the second network comprises a part or all of the second network slice information; wherein the first network and the second network have different access network devices and/or core network devices; and wherein either the first network is fourth generation mobile communications system, 4G network and the second network is a fifth generation mobile communications system, 5G network or the first network is fifth generation mobile communications system, 5G network and the second network is a fourth generation mobile communications system, 4G network. <EOS>