An air interface synchronization method, comprising: listening to, by an unsynchronized indoor base station, synchronization sub-frames transmitted from an outdoor base station and synchronization sub-frames transmitted from at least one indoor base station synchronized with the outdoor base station and selecting, by the unsynchronized indoor base station, a synchronization reference base station from the base stations transmitting the synchronization sub-frames heard by the unsynchronized indoor base station, wherein each of the synchronization sub-frames comprises a dedicated synchronization channel over which a synchronization sequence is broadcast; and acquiring, by the unsynchronized indoor base station, the synchronization sequence broadcast over the dedicated synchronization channel which is set by the synchronization reference base station in its transmitted synchronization sub-frame and synchronizing with the synchronization reference base station according to the synchronization sequence; wherein the outdoor base station and the at least one indoor base station synchronized with the outdoor base station broadcast their own priority information in their transmitted synchronization sub-frames, wherein the priority information is clock level information; and selecting by the unsynchronized indoor base station the synchronization reference base station from the base stations transmitting the heard synchronization sub-frames comprises: acquiring by the unsynchronized indoor base station the priority information of the base stations transmitting the heard synchronization sub-frames and selecting the synchronization reference base station from the base stations transmitting the heard synchronization sub-frames according to the priority information; and the method further comprises: after the unsynchronized indoor base station selects the synchronization reference base station from the base stations transmitting the heard synchronization sub-frames according to the priority information, setting by the unsynchronized indoor base station its own priority information below the priority information of the synchronization reference base station. listening to, by an unsynchronized indoor base station, synchronization sub-frames transmitted from an outdoor base station and synchronization sub-frames transmitted from at least one indoor base station synchronized with the outdoor base station and selecting, by the unsynchronized indoor base station, a synchronization reference base station from the base stations transmitting the synchronization sub-frames heard by the unsynchronized indoor base station, wherein each of the synchronization sub-frames comprises a dedicated synchronization channel over which a synchronization sequence is broadcast; and acquiring, by the unsynchronized indoor base station, the synchronization sequence broadcast over the dedicated synchronization channel which is set by the synchronization reference base station in its transmitted synchronization sub-frame and synchronizing with the synchronization reference base station according to the synchronization sequence; wherein the outdoor base station and the at least one indoor base station synchronized with the outdoor base station broadcast their own priority information in their transmitted synchronization sub-frames, wherein the priority information is clock level information; and selecting by the unsynchronized indoor base station the synchronization reference base station from the base stations transmitting the heard synchronization sub-frames comprises: acquiring by the unsynchronized indoor base station the priority information of the base stations transmitting the heard synchronization sub-frames and selecting the synchronization reference base station from the base stations transmitting the heard synchronization sub-frames according to the priority information; and the method further comprises: after the unsynchronized indoor base station selects the synchronization reference base station from the base stations transmitting the heard synchronization sub-frames according to the priority information, setting by the unsynchronized indoor base station its own priority information below the priority information of the synchronization reference base station. acquiring by the unsynchronized indoor base station the priority information of the base stations transmitting the heard synchronization sub-frames and selecting the synchronization reference base station from the base stations transmitting the heard synchronization sub-frames according to the priority information; and the method further comprises: after the unsynchronized indoor base station selects the synchronization reference base station from the base stations transmitting the heard synchronization sub-frames according to the priority information, setting by the unsynchronized indoor base station its own priority information below the priority information of the synchronization reference base station.