Embodiments of the present invention provide a method, a device, and a communications system for performing data communication by using an unlicensed spectrum. In an embodiment, a base station determines to use an unlicensed spectrum to perform data communication, and sends mode request information to a terminal. The terminal receives the mode request message sent by the base station and determines, according to a preset configuration table, whether to use the unlicensed spectrum to perform data communication. When determining to use the unlicensed spectrum to perform data communication, the terminal sends a mode acknowledgement message to the base station. The base station receives the mode acknowledgement information sent by the terminal, and the terminal performs data communication in a first unlicensed frequency band with the base station.