A downlink data transmission method, wherein the method comprises: receiving, by a network device, carrier aggregation CA capability indication information sent by a terminal device, wherein the CA capability indication information is used to indicate, to the network device, that the terminal device supports at least one first-type intra-band non-contiguous carrier aggregation intra-band NC CA combination on a first frequency band, and a maximum downlink spectrum width supported by the terminal device; determining, by the network device based on the CA capability indication information, a first CA combination from the at least one first-type intra-band NC CA combination of the first frequency band, wherein the first CA combination comprises at least two component carriers CCs, and wherein the determining, by the network device based on the CA capability indication information, the first CA combination from the at least one first-type intra-band NC CA of the first frequency band specifically comprises determining, by the network device, a maximum occupied spectrum width based on a spectrum width occupied by each first-type intra-band NC CA combination, and determining, by the network device, the first CA combination based on the maximum occupied spectrum width and the maximum downlink spectrum width supported by the terminal device; sending, by the network device, downlink data to the terminal device by using the first CA combination; and characterized in that: the determining, by the network device, the first CA combination based on the maximum occupied spectrum width and the maximum downlink spectrum width supported by the terminal device specifically comprises: determining, by the network device based on the maximum occupied spectrum width and the maximum downlink spectrum width supported by the terminal device, that the maximum downlink spectrum width supported by the terminal device is less than the maximum occupied spectrum width; determining, by the network device from the at least one first-type intra-band NC CA combination of the first frequency band, at least one second available CA combination whose spectrum width is less than or equal to the maximum downlink spectrum width; and determining, by the network device, the first CA combination based on the at least one second available CA combination, wherein the at least one second available CA combination comprises the first CA combination. receiving, by a network device, carrier aggregation CA capability indication information sent by a terminal device, wherein the CA capability indication information is used to indicate, to the network device, that the terminal device supports at least one first-type intra-band non-contiguous carrier aggregation intra-band NC CA combination on a first frequency band, and a maximum downlink spectrum width supported by the terminal device; determining, by the network device based on the CA capability indication information, a first CA combination from the at least one first-type intra-band NC CA combination of the first frequency band, wherein the first CA combination comprises at least two component carriers CCs, and wherein the determining, by the network device based on the CA capability indication information, the first CA combination from the at least one first-type intra-band NC CA of the first frequency band specifically comprises determining, by the network device, a maximum occupied spectrum width based on a spectrum width occupied by each first-type intra-band NC CA combination, and determining, by the network device, the first CA combination based on the maximum occupied spectrum width and the maximum downlink spectrum width supported by the terminal device; sending, by the network device, downlink data to the terminal device by using the first CA combination; and characterized in that: the determining, by the network device, the first CA combination based on the maximum occupied spectrum width and the maximum downlink spectrum width supported by the terminal device specifically comprises: determining, by the network device based on the maximum occupied spectrum width and the maximum downlink spectrum width supported by the terminal device, that the maximum downlink spectrum width supported by the terminal device is less than the maximum occupied spectrum width; determining, by the network device from the at least one first-type intra-band NC CA combination of the first frequency band, at least one second available CA combination whose spectrum width is less than or equal to the maximum downlink spectrum width; and determining, by the network device, the first CA combination based on the at least one second available CA combination, wherein the at least one second available CA combination comprises the first CA combination. determining, by the network device based on the maximum occupied spectrum width and the maximum downlink spectrum width supported by the terminal device, that the maximum downlink spectrum width supported by the terminal device is less than the maximum occupied spectrum width; determining, by the network device from the at least one first-type intra-band NC CA combination of the first frequency band, at least one second available CA combination whose spectrum width is less than or equal to the maximum downlink spectrum width; and determining, by the network device, the first CA combination based on the at least one second available CA combination, wherein the at least one second available CA combination comprises the first CA combination.