Provided are a data transmission method and apparatus, a terminal device, and a storage medium. The method includes determining a basic unit carrying client data, where the rate of the client data is smaller than the set value, the set value is determined according to the bandwidth of a flexible Ethernet (FlexE) slot, and the basic unit is a basic unit included in a basic unit set; mapping the client data to the basic unit; and sending, through the FlexE slot, the basic unit set to which the client data is mapped.