A data transmission method, comprising: sending, by an access network device, first control information to a first terminal device, wherein the first control information is used to instruct the first terminal device to transmit downlink data received from the access network device, to a second terminal device a plurality of times through a sidelink; and sending, by the access network device, the downlink data to the first terminal device, wherein the first control information comprises a coordinated transmission instruction, first resource information, and second resource information; and the coordinated transmission instruction is used to instruct the first terminal device to transmit the downlink data received by the first terminal device based on the first resource information, to the second terminal device the plurality of times through the sidelink based on the second resource information characterized in that the data transmission method is performed for ultra-reliable and low latency communications, URLLC, transmission in a 5G communications system, wherein the method further comprises: forming, by the access network device, the second resource information based on a to-be-cancelled uplink scheduling resource, or forming, by the access network device, the second resource information based on a new uplink resource allocated by the access network device for the second terminal device, wherein the new uplink resource is different from an uplink resource that has been scheduled by the access network device. sending, by an access network device, first control information to a first terminal device, wherein the first control information is used to instruct the first terminal device to transmit downlink data received from the access network device, to a second terminal device a plurality of times through a sidelink; and sending, by the access network device, the downlink data to the first terminal device, wherein the first control information comprises a coordinated transmission instruction, first resource information, and second resource information; and the coordinated transmission instruction is used to instruct the first terminal device to transmit the downlink data received by the first terminal device based on the first resource information, to the second terminal device the plurality of times through the sidelink based on the second resource information characterized in that the data transmission method is performed for ultra-reliable and low latency communications, URLLC, transmission in a 5G communications system, wherein the method further comprises: forming, by the access network device, the second resource information based on a to-be-cancelled uplink scheduling resource, or forming, by the access network device, the second resource information based on a new uplink resource allocated by the access network device for the second terminal device, wherein the new uplink resource is different from an uplink resource that has been scheduled by the access network device. forming, by the access network device, the second resource information based on a to-be-cancelled uplink scheduling resource, or forming, by the access network device, the second resource information based on a new uplink resource allocated by the access network device for the second terminal device, wherein the new uplink resource is different from an uplink resource that has been scheduled by the access network device.