A transmission device and an antenna down-tilt control system includes: an input component, comprising an inner gear ring, a first gear and a second gear both coaxial with the inner gear ring, and a third gear engaged with the first gear; a unidirectional control mechanism, comprising a first unidirectional component, the first unidirectional component comprising a first rotating member that can unidirectionally rotate in a first rotation direction, the first rotating member being fixedly connected to the inner gear ring; an output component, comprising at least two fourth gears arranged at an interval in a circumferential direction, the third gear being selectively engaged with any one of the fourth gears; and a rotary component, comprising a fifth gear and a rotary box.