An S1 message transmission method performed by an intermediate device, wherein the intermediate device is configured between a core network device and a small cell set, comprising: obtaining, by the intermediate device, a first S1 message, wherein the first S1 message comprises a macro base station identity and a cell global identity, and the macro base station identity is an identity of the intermediate device; determining, by the intermediate device, whether the first S1 message needs to be sent to a small cell in the small cell set managed by the intermediate device; if the first S1 message needs to be sent to the small cell in the small cell set managed by the intermediate device, obtaining, by the intermediate device, a small cell identity corresponding to the cell global identity, converting the macro base station identity to the small cell identity, and generating a second S1 message, wherein the second S1 message comprises the small cell identity; and sending, by the intermediate device, the second S1 message to a small cell corresponding to the small cell identity; wherein, a bit width of the macro base station identity is less than or equal to 20 bits and a bit width of the small cell identity is greater than 20 bits; wherein the obtaining, by an intermediate device, the first S1 message comprises: receiving, by the intermediate device, the first S1 message sent by a core network device; wherein the converting the macro base station identity to the small cell identity comprises one of: converting the macro base station identity to the small cell identity according to a field of the macro base station identity and an idle field of an identity other than the macro base station identity, wherein the idle field has a bit width equal to the difference between the bit width of the macro base station identity and the bit width of the small cell identity; and converting the macro base station identity to the small cell identity by extending a custom field of the macro base station identity so that the resulting bit width of the macro base station identity is equal to the bit width of the small cell identity. obtaining, by the intermediate device, a first S1 message, wherein the first S1 message comprises a macro base station identity and a cell global identity, and the macro base station identity is an identity of the intermediate device; determining, by the intermediate device, whether the first S1 message needs to be sent to a small cell in the small cell set managed by the intermediate device; if the first S1 message needs to be sent to the small cell in the small cell set managed by the intermediate device, obtaining, by the intermediate device, a small cell identity corresponding to the cell global identity, converting the macro base station identity to the small cell identity, and generating a second S1 message, wherein the second S1 message comprises the small cell identity; and sending, by the intermediate device, the second S1 message to a small cell corresponding to the small cell identity; wherein, a bit width of the macro base station identity is less than or equal to 20 bits and a bit width of the small cell identity is greater than 20 bits; wherein the obtaining, by an intermediate device, the first S1 message comprises: receiving, by the intermediate device, the first S1 message sent by a core network device; wherein the converting the macro base station identity to the small cell identity comprises one of: converting the macro base station identity to the small cell identity according to a field of the macro base station identity and an idle field of an identity other than the macro base station identity, wherein the idle field has a bit width equal to the difference between the bit width of the macro base station identity and the bit width of the small cell identity; and converting the macro base station identity to the small cell identity by extending a custom field of the macro base station identity so that the resulting bit width of the macro base station identity is equal to the bit width of the small cell identity. receiving, by the intermediate device, the first S1 message sent by a core network device; wherein the converting the macro base station identity to the small cell identity comprises one of: converting the macro base station identity to the small cell identity according to a field of the macro base station identity and an idle field of an identity other than the macro base station identity, wherein the idle field has a bit width equal to the difference between the bit width of the macro base station identity and the bit width of the small cell identity; and converting the macro base station identity to the small cell identity by extending a custom field of the macro base station identity so that the resulting bit width of the macro base station identity is equal to the bit width of the small cell identity. converting the macro base station identity to the small cell identity according to a field of the macro base station identity and an idle field of an identity other than the macro base station identity, wherein the idle field has a bit width equal to the difference between the bit width of the macro base station identity and the bit width of the small cell identity; and converting the macro base station identity to the small cell identity by extending a custom field of the macro base station identity so that the resulting bit width of the macro base station identity is equal to the bit width of the small cell identity.