This application relates to the communications field, and in particular, to a data transmission method, an apparatus, and a system. In the data transmission method, a base station determines a transmission policy, where the transmission policy is used for contention-based uplink data transmission or non-contention based uplink data transmission; and sends the transmission policy to a terminal. The terminal receives the transmission policy sent by the base station, and sends uplink data to the base station based on the transmission policy. With a solution in this application, the base station can determine an appropriate transmission policy based on a network status, and send the transmission policy to the terminal, and the terminal can use, based on the transmission policy, an appropriate transmission mode to send uplink data to the base station, so that transmission efficiency is improved.