Provided are a data transmission method and system for when an LTE system operates using a time division duplexing mode in an unlicensed frequency band, and a device having a base station function. The data transmission method comprises: a channel monitoring sub-frame used for periodically detecting a downlink channel state is arranged in a frame structure of a time division duplexing mode; within any period, if the channel monitoring sub-frame detects that a downlink channel is in idle state, downlink data is sent via a downlink sub-frame located within the period in the frame structure, and if not, downlink data is not sent within the period.