A short-axle large-speed-ratio gear set structure, comprising: a housing; a transmission axle, which is rotatably mounted in the housing, the transmission axle being provided with a sun gear; an internally-toothed ring track, which is fixed to the housing; a planetary gear set, which comprises a first carrier frame and a second carrier frame that are opposite to each other, a plurality of stepped planetary gears being rotatably arranged between the first and second carrier frames (31, 35) and around an outer circumference of the sun gear, each of the stepped planetary gears comprising a first toothed portion synchronously meshing with the sun gear and the internally-toothed ring track, each of the stepped planetary gears further comprising a second toothed portion that is integrated and rotatable with the first toothed portion; and an output member, which has an inner circumference that is formed with an internal toothed circumference meshing with the second toothed portion of each of the stepped planetary gears, the output member having an outer circumference that is formed with a driving wheel portion for driving purposes, characterized in that the second carrier frame of the planetary gear set is provided, at a center thereof, with an axle tube, the axle tube rotatably supporting the output member by means of a third bearing, wherein the axle tube of the second carrier frame is provided, at a center thereof, with a stepped mounting hole that receives a mounting axle to penetrate therein, one end of the mounting axle being rotatably mounted on an end of the sun gear, wherein the mounting axle is formed with a through hole, and wherein the first and second carrier frames (31, 35) of the planetary gear set are rotatably mounted, by means of a first bearing and a second bearing arranged at centers thereof, to two ends of the sun gear, such that the sun gear, the planetary gear set, and the output member are assembled together as a coaxial gear set. a housing; a transmission axle, which is rotatably mounted in the housing, the transmission axle being provided with a sun gear; an internally-toothed ring track, which is fixed to the housing; a planetary gear set, which comprises a first carrier frame and a second carrier frame that are opposite to each other, a plurality of stepped planetary gears being rotatably arranged between the first and second carrier frames (31, 35) and around an outer circumference of the sun gear, each of the stepped planetary gears comprising a first toothed portion synchronously meshing with the sun gear and the internally-toothed ring track, each of the stepped planetary gears further comprising a second toothed portion that is integrated and rotatable with the first toothed portion; and an output member, which has an inner circumference that is formed with an internal toothed circumference meshing with the second toothed portion of each of the stepped planetary gears, the output member having an outer circumference that is formed with a driving wheel portion for driving purposes, characterized in that the second carrier frame of the planetary gear set is provided, at a center thereof, with an axle tube, the axle tube rotatably supporting the output member by means of a third bearing, wherein the axle tube of the second carrier frame is provided, at a center thereof, with a stepped mounting hole that receives a mounting axle to penetrate therein, one end of the mounting axle being rotatably mounted on an end of the sun gear, wherein the mounting axle is formed with a through hole, and wherein the first and second carrier frames (31, 35) of the planetary gear set are rotatably mounted, by means of a first bearing and a second bearing arranged at centers thereof, to two ends of the sun gear, such that the sun gear, the planetary gear set, and the output member are assembled together as a coaxial gear set.