A data transmission method, a device, and a communications system are described. The method includes generating, by a terminal device, first indication information. The first indication information is used to indicate logical channel groups having to-be-sent data and amount of to-be-sent data on part of the logical channel groups having to-be-sent data. The method includes sending, by the terminal device, the first indication information. According to the data transmission method, the device, and the communications system provided in the described embodiments, when a current quantity idle bits in a MAC PDU is insufficient, the terminal device may indicate, to a network device by using the first indication information, the LCGs having to-be-sent data on the terminal device. In this way, uplink transmission resource allocation improved efficiency.