The invention provides a device with a base station function and a method and a system for data transmission when an LTE (long-term evolution) system works in an unauthorized frequency band. The data transmission method includes: setting a channel monitoring subframe for periodically detecting the state of a downlink channel in a frame structure of the LTE system; within any period, if the channel monitoring subframe detects that the downlink channel is in the idle state, continuing detecting the state of the downlink state within set back-off time; if the downlink channel is detected to be in the idle state still within the back-off time, sending downlink data through a downlink subframe within any period. Under the premise that the LTE system works normally in the unauthorized frequency band, large interference on other systems with interference avoidance mechanisms when the LTE system works in the unauthorized frequency band is avoided, and the problem that the devices with the base station function belonging to different operators send data simultaneously resulting in collision due to the fact that channel measurement is performed at the same time is avoided as well.