A method for data transmission, comprising: receiving, by a terminal device, a deactivation instruction from a network device, wherein the deactivation instruction is used for deactivating a duplicated-data transmission function of a radio bearer; in response to the deactivation instruction, determining, by the terminal device, to change a transmission mode of a packet data convergence protocol, PDCP, protocol data unit, PDU, to a split-bearer mode and to change a radio link control, RLC, entity for transmitting the PDCP PDU, wherein RLC entities associated with the radio bearer comprise at least three RLC entities; and determining, by the terminal device, a target RLC entity for transmitting the PDCP PDU; after determining, by the terminal device, to change the RLC entity for transmitting the PDCP PDU in response to the deactivation instruction, the method further comprises: receiving, by the terminal device, configuration signaling from the network device, wherein the configuration signaling is radio resource control, RRC, signaling, configuration of an RLC bearer of the configuration signaling contains an indication field indicative of which of the at least three RLC entities is a primary RLC entity and which of the at least three RLC entities is a secondary RLC entity; the method further comprises: determining, by the terminal device, the target RLC entity according to a data volume of a PDCP layer and a total data volume of an RLC layer associated with the radio bearer, comprises: determining the primary RLC entity or the secondary RLC entity as the target RLC entity, in response to a sum of the data volume of the PDCP layer and the total data volume of the RLC layer associated with the radio bearer being larger than or equal to a preset threshold; or determining the primary RLC entity as the target RLC entity, in response to the sum of the data volume of the PDCP layer and the total data volume of the RLC layer associated with the radio bearer being smaller than the preset threshold. receiving, by a terminal device, a deactivation instruction from a network device, wherein the deactivation instruction is used for deactivating a duplicated-data transmission function of a radio bearer; in response to the deactivation instruction, determining, by the terminal device, to change a transmission mode of a packet data convergence protocol, PDCP, protocol data unit, PDU, to a split-bearer mode and to change a radio link control, RLC, entity for transmitting the PDCP PDU, wherein RLC entities associated with the radio bearer comprise at least three RLC entities; and determining, by the terminal device, a target RLC entity for transmitting the PDCP PDU; after determining, by the terminal device, to change the RLC entity for transmitting the PDCP PDU in response to the deactivation instruction, the method further comprises: receiving, by the terminal device, configuration signaling from the network device, wherein the configuration signaling is radio resource control, RRC, signaling, configuration of an RLC bearer of the configuration signaling contains an indication field indicative of which of the at least three RLC entities is a primary RLC entity and which of the at least three RLC entities is a secondary RLC entity; the method further comprises: determining, by the terminal device, the target RLC entity according to a data volume of a PDCP layer and a total data volume of an RLC layer associated with the radio bearer, comprises: determining the primary RLC entity or the secondary RLC entity as the target RLC entity, in response to a sum of the data volume of the PDCP layer and the total data volume of the RLC layer associated with the radio bearer being larger than or equal to a preset threshold; or determining the primary RLC entity as the target RLC entity, in response to the sum of the data volume of the PDCP layer and the total data volume of the RLC layer associated with the radio bearer being smaller than the preset threshold. determining the primary RLC entity or the secondary RLC entity as the target RLC entity, in response to a sum of the data volume of the PDCP layer and the total data volume of the RLC layer associated with the radio bearer being larger than or equal to a preset threshold; or determining the primary RLC entity as the target RLC entity, in response to the sum of the data volume of the PDCP layer and the total data volume of the RLC layer associated with the radio bearer being smaller than the preset threshold.