A data transmission apparatus, wherein the apparatus is applied to a source base station, and the apparatus comprises: a processing unit, configured to determine first data and a radio air interface protocol sequence number of a data unit used to carry the first data, wherein the first data comprises data that is not acknowledged by a terminal for correct reception during data transmission between the source base station and the terminal; and a sending unit, configured to send the first data and the radio air interface protocol sequence number to a target base station; wherein a core network connected to the target base station is different from a core network connected to the source base station, and the source base station and the target base station are a source base station and a target base station of the terminal in a handover process between different core networks, wherein the apparatus further comprising a receiving unit, wherein the processing unit is further configured to: before the source base station sends the first data and the radio air interface protocol sequence number to the target base station, configure first radio bearer information for the terminal, wherein the first radio bearer information is used to establish a radio bearer between the source base station and the terminal; the sending unit is further configured to send the first radio bearer information to the target base station; the receiving unit is configured to receive an acknowledgment message fed back by the target base station, wherein the acknowledgment message is used to indicate second radio bearer information between the target base station and the terminal, the second radio bearer information is determined by the target base station based on the first radio bearer information and includes configuration information of a radio bearer used to carry the first data, wherein the acknowledgment message is further used to indicate third radio bearer information, the third radio bearer information comprises configuration information for configuring third data, and the third data is data exchanged between the terminal and the core network connected to the target base station; and the sending unit is further configured to send the second and the third radio bearer information to the terminal. a processing unit, configured to determine first data and a radio air interface protocol sequence number of a data unit used to carry the first data, wherein the first data comprises data that is not acknowledged by a terminal for correct reception during data transmission between the source base station and the terminal; and a sending unit, configured to send the first data and the radio air interface protocol sequence number to a target base station; wherein a core network connected to the target base station is different from a core network connected to the source base station, and the source base station and the target base station are a source base station and a target base station of the terminal in a handover process between different core networks, wherein the apparatus further comprising a receiving unit, wherein the processing unit is further configured to: before the source base station sends the first data and the radio air interface protocol sequence number to the target base station, configure first radio bearer information for the terminal, wherein the first radio bearer information is used to establish a radio bearer between the source base station and the terminal; the sending unit is further configured to send the first radio bearer information to the target base station; the receiving unit is configured to receive an acknowledgment message fed back by the target base station, wherein the acknowledgment message is used to indicate second radio bearer information between the target base station and the terminal, the second radio bearer information is determined by the target base station based on the first radio bearer information and includes configuration information of a radio bearer used to carry the first data, wherein the acknowledgment message is further used to indicate third radio bearer information, the third radio bearer information comprises configuration information for configuring third data, and the third data is data exchanged between the terminal and the core network connected to the target base station; and the sending unit is further configured to send the second and the third radio bearer information to the terminal.