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 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 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 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 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 . <EOS>