Disclosed are a data transmission method and apparatus, a terminal device, and a storage medium. The data transmission method includes: determining a basic unit carrying client data, wherein a rate of the client data is less than a set value determined according to a rate of a carrying container of an optical transport network (OTN) frame, and the basic unit is a basic unit contained in a basic unit set; mapping the client data to the basic unit; and sending, through the OTN frame, the basic unit set to which the client data is mapped.