The invention discloses a robot sucking disc leg structure based on a fixed-axis gear train. The robot sucking disc leg structure comprises a large sucking disc group and a small sucking disc group and is characterized in that the large sucking disc group comprises a large sucking disc (1), an upper rotary knob (15), a first control mechanism, a large shell (11) and a medium shell (8); and the small sucking disc group comprises a small sucking disc (5), a lower rotary knob (14), a small shell (3), a reinforcing post (7), a large gear (9), a small gear (10) and a second control mechanism. The sucking disc leg structure adopts the fixed-axis gear train as a transmission system; and the small sucking disc acts on the large sucking disc. According to the robot sucking disc leg structure, sucking force of the sucking disc leg is generated by utilizing a vacuum degree; contamination and damage cannot be generated on an acting surface; the robot sucking disc leg structure can walk on an irregular rough working surface, so that the application range of a robot using the sucking disc leg is greatly expanded; and the self-adaption and sucking force of the robot on the rough surface are increased.