A data transmission method and an apparatus are provided. The data transmission method in the present invention includes: determining, by a first device and according to data stream information, whether a data transfer protocol TCP parameter in a data stream corresponding to the data stream information needs to be modified, where the data stream information includes at least one of the following information: information about the first device, information about a second device, service information, or network status information; if determining that the TCP parameter needs to be modified, modifying, by the first device, the TCP parameter, to obtain a modified TCP parameter; and generating, by the first device, a modification result of the TCP parameter.