A data transmission method, comprising the following steps: receiving a first downlink data packet sent by a network device, the first downlink data packet comprising a first reflective QoS indication RQI that is a first preset value; keeping, according to the first preset value, a mapping rule of a first mapping relationship and a mapping rule of a second mapping relationship to be both unchanged, and receiving an radio resource control RRC message sent by the network device, the first mapping relationship being a mapping relationship from internet protocol IP flows to quality of service QoS flows, the second mapping relationship being a mapping relationship from QoS flows to data radio bearers DRBs, and the RRC message comprising a target mapping rule; and determining, according to the RRC message, the mapping rule of the second mapping relationship, as the target mapping rule, characterised by before receiving the first RQI sent by the network device, the method further comprises the steps of: receiving a second downlink data packet sent by the network device, the second downlink data packet comprising a second RQI that is a second preset value; activating, according to the second preset value, a first reflective QoS to update the first mapping relationship, and activating, according to the second preset value, a second reflective QoS to update the second mapping relationship, the updating the first mapping relationship comprising determining the first mapping relationship of the second downlink data packet as the first mapping relationship of a second uplink data packet, and the updating the second mapping relationship comprising determining the second mapping relationship of the second downlink data packet as the second mapping relationship of the second uplink data packet; sending the second uplink data packet to the network device; keeping, according to the first preset value, the mapping rule of the first mapping relationship and the mapping rule of the second mapping relationship to be both unchanged comprises: keeping an activation state of the first reflective QoS, and an activation state of the second reflective QoS. receiving a first downlink data packet sent by a network device, the first downlink data packet comprising a first reflective QoS indication RQI that is a first preset value; keeping, according to the first preset value, a mapping rule of a first mapping relationship and a mapping rule of a second mapping relationship to be both unchanged, and receiving an radio resource control RRC message sent by the network device, the first mapping relationship being a mapping relationship from internet protocol IP flows to quality of service QoS flows, the second mapping relationship being a mapping relationship from QoS flows to data radio bearers DRBs, and the RRC message comprising a target mapping rule; and determining, according to the RRC message, the mapping rule of the second mapping relationship, as the target mapping rule, characterised by before receiving the first RQI sent by the network device, the method further comprises the steps of: receiving a second downlink data packet sent by the network device, the second downlink data packet comprising a second RQI that is a second preset value; activating, according to the second preset value, a first reflective QoS to update the first mapping relationship, and activating, according to the second preset value, a second reflective QoS to update the second mapping relationship, the updating the first mapping relationship comprising determining the first mapping relationship of the second downlink data packet as the first mapping relationship of a second uplink data packet, and the updating the second mapping relationship comprising determining the second mapping relationship of the second downlink data packet as the second mapping relationship of the second uplink data packet; sending the second uplink data packet to the network device; keeping, according to the first preset value, the mapping rule of the first mapping relationship and the mapping rule of the second mapping relationship to be both unchanged comprises: keeping an activation state of the first reflective QoS, and an activation state of the second reflective QoS. receiving a second downlink data packet sent by the network device, the second downlink data packet comprising a second RQI that is a second preset value; activating, according to the second preset value, a first reflective QoS to update the first mapping relationship, and activating, according to the second preset value, a second reflective QoS to update the second mapping relationship, the updating the first mapping relationship comprising determining the first mapping relationship of the second downlink data packet as the first mapping relationship of a second uplink data packet, and the updating the second mapping relationship comprising determining the second mapping relationship of the second downlink data packet as the second mapping relationship of the second uplink data packet; sending the second uplink data packet to the network device; keeping, according to the first preset value, the mapping rule of the first mapping relationship and the mapping rule of the second mapping relationship to be both unchanged comprises: keeping an activation state of the first reflective QoS, and an activation state of the second reflective QoS.