Abstract: An actuator and a transmission structure thereof. The transmission structure (3) includes a screw nut (31) and a ratchet guiding sleeve (32). The screw nut (31) includes one or a plurality of pawls (311). The ratchet guiding sleeve (32) is mounted on the screw nut (31), and the screw nut (31) is able to rotate relative to the ratchet guiding sleeve (32). The ratchet guiding sleeve (32) includes a ring ratchet (321) disposed on an inner circumference thereof and a plurality of track slots (322) disposed on an outer circumference thereof. One or a plurality of pawls (311) abut against the ring ratchet (321), thereby achieving the effect of smooth movements of the actuator (10) and the transmission structure (3).