A network device, wherein the network device is applied to data transmission in a preset spectrum, the preset spectrum is a spectrum that occupies a spectrum resource for a limited time each time, and the network device comprises: a control unit, configured to write a data retransmission identifier of a first carrier into a preset field of a physical downlink control channel, wherein the preset field comprises a carrier indicator, CIF, field and a new data indicator, NDI, field, or the preset field comprises a CIF field, an NDI field and a resource block assignment, RBA, field, and wherein the first carrier is a carrier in the preset spectrum; wherein the control unit is further configured to generate a data scheduling instruction of the physical downlink control channel, wherein the data scheduling instruction of the physical downlink control channel is used to instruct to retransmit data that is on the first carrier; and a sending unit, configured to send, on a second carrier, the data scheduling instruction of the physical downlink control channel generated by the control unit to a receiving device; wherein the sending unit is further configured to send the data that is on the first carrier on the second carrier according to the data scheduling instruction of the physical downlink control channel; and wherein the control unit is specifically configured to: when the preset field comprises the CIF field, use a first index number of the first carrier as the data retransmission identifier of the first carrier and write the first index number of the first carrier into the CIF field, wherein the first index number of the first carrier is used to instruct to retransmit the data of the first carrier on the second carrier; and the control unit is specifically configured to: when the preset field comprises the NDI field, use a non-new data identifier as the data retransmission identifier of the first carrier and write the non-new data identifier into the NDI field, wherein the non-new data identifier is used to instruct to retransmit the data of the first carrier on the second carrier. a control unit, configured to write a data retransmission identifier of a first carrier into a preset field of a physical downlink control channel, wherein the preset field comprises a carrier indicator, CIF, field and a new data indicator, NDI, field, or the preset field comprises a CIF field, an NDI field and a resource block assignment, RBA, field, and wherein the first carrier is a carrier in the preset spectrum; wherein the control unit is further configured to generate a data scheduling instruction of the physical downlink control channel, wherein the data scheduling instruction of the physical downlink control channel is used to instruct to retransmit data that is on the first carrier; and a sending unit, configured to send, on a second carrier, the data scheduling instruction of the physical downlink control channel generated by the control unit to a receiving device; wherein the sending unit is further configured to send the data that is on the first carrier on the second carrier according to the data scheduling instruction of the physical downlink control channel; and wherein the control unit is specifically configured to: when the preset field comprises the CIF field, use a first index number of the first carrier as the data retransmission identifier of the first carrier and write the first index number of the first carrier into the CIF field, wherein the first index number of the first carrier is used to instruct to retransmit the data of the first carrier on the second carrier; and the control unit is specifically configured to: when the preset field comprises the NDI field, use a non-new data identifier as the data retransmission identifier of the first carrier and write the non-new data identifier into the NDI field, wherein the non-new data identifier is used to instruct to retransmit the data of the first carrier on the second carrier.