A coaxial gear set reduction mechanism, comprising: a housing; a transmission axle, which is rotatably mounted in the housing; an internally toothed ring track, which is fixed to the housing; a coaxial gear set, which comprises a sun gear, a planetary gear set, and an output member, wherein the sun gear is selectively mountable to and connected to the transmission axle to rotate together therewith; the planetary gear set is arranged, in a rotatable manner, between two ends of the sun gear, and the planetary gear set is simultaneously in meshing engagement with the sun gear and the internally toothed ring track; and the output member is arranged at one side of the planetary gear set that is opposite to the internally toothed ring track, and the output member is driven by the planetary gear set, characterized in that one end of the transmission axle is formed with a mounting section and a coupling section, and the sun gear of the coaxial gear set is formed, in a center thereof, with a coupling hole that is fit over the mounting section, a coupling block being inserted into and disposed in the coupling hole to engage with the coupling section of the transmission axle, a fastening member penetrating through and pressing on the coupling block and fastened to the transmission axle, wherein the planetary gear set of the coaxial gear set comprises a first carrier frame and a second carrier frame that are opposite to each other and are respectively and rotatably arranged at two ends of the sun gear, a plurality of 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 planetary gears comprising a first toothed portion synchronously meshing with the sun gear and the internally toothed ring track, each of the planetary gears further comprising a second toothed portion meshing with the output member, the output member being rotatably mounted on the second carrier frame of the planetary gear set. a housing; a transmission axle, which is rotatably mounted in the housing; an internally toothed ring track, which is fixed to the housing; a coaxial gear set, which comprises a sun gear, a planetary gear set, and an output member, wherein the sun gear is selectively mountable to and connected to the transmission axle to rotate together therewith; the planetary gear set is arranged, in a rotatable manner, between two ends of the sun gear, and the planetary gear set is simultaneously in meshing engagement with the sun gear and the internally toothed ring track; and the output member is arranged at one side of the planetary gear set that is opposite to the internally toothed ring track, and the output member is driven by the planetary gear set, characterized in that one end of the transmission axle is formed with a mounting section and a coupling section, and the sun gear of the coaxial gear set is formed, in a center thereof, with a coupling hole that is fit over the mounting section, a coupling block being inserted into and disposed in the coupling hole to engage with the coupling section of the transmission axle, a fastening member penetrating through and pressing on the coupling block and fastened to the transmission axle, wherein the planetary gear set of the coaxial gear set comprises a first carrier frame and a second carrier frame that are opposite to each other and are respectively and rotatably arranged at two ends of the sun gear, a plurality of 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 planetary gears comprising a first toothed portion synchronously meshing with the sun gear and the internally toothed ring track, each of the planetary gears further comprising a second toothed portion meshing with the output member, the output member being rotatably mounted on the second carrier frame of the planetary gear set.