A planetary speed-change mechanism, comprising: an output shaft; a transmission module, which is mounted on the output shaft, the transmission module comprising a driving roller that is operable to drive the output shaft in a single direction; and at least one speed step-up module, which is mounted on the output shaft, the speed step-up module comprising a connecting gear, a planet speed-change wheel assembly, a speed step-up gear, and an arrestor assembly, wherein the connecting gear is rotatably mounted on the output shaft and is in operative coupling with the driving roller; the speed step-up gear is fixedly mounted to the output shaft; and the planet speed-change wheel assembly comprises at least one speed-change gear, each of the at least one speed-change gear comprising a small-diameter toothed portion in mating engagement with the connecting gear and a large-diameter toothed portion in mating engagement with the speed step-up gear; wherein in a low-speed position, the planet speed-change wheel assembly of the speed step-up module is in idling rotation as being set in an orbiting motion so that an input of power is supplied through the driving roller to drive the output shaft in the single direction; and in a high-speed position, the arrestor assembly is activated to stop the orbiting motion of the planet speed-change wheel assembly, so as to have the speed-change gear of the planet speed-change wheel assembly switching from the orbiting motion relative to the speed step-up gear to a spinning motion to step the speed up, wherein the output shaft comprises a shaft expansion section, the driving roller of the transmission module being mounted by means of a one-way shaft assembly to the shaft expansion section, and the driving roller is formed, in a center thereof, with a shaft compartment in which the one-way shaft assembly is arranged, so that the driving roller is operable to drive the output shaft in the single direction and is in idling rotation in an opposite direction; and the planet speed-change wheel assembly comprises two or more than two speed-change gears that are sequentially in engagement with each other and a terminal one of the two or more than two speed-change gears is in mating engagement with the speed step-up gear to improve speed stepping up, characterized in that the driving roller of the transmission module is formed, in a center thereof, with an engaging portion, with which an engaging portion of the connecting gear of the speed step-up module is in mating engagement to have the connecting gear of the speed step-up module and the driving roller moving in synchronization with each other. an output shaft; a transmission module, which is mounted on the output shaft, the transmission module comprising a driving roller that is operable to drive the output shaft in a single direction; and at least one speed step-up module, which is mounted on the output shaft, the speed step-up module comprising a connecting gear, a planet speed-change wheel assembly, a speed step-up gear, and an arrestor assembly, wherein the connecting gear is rotatably mounted on the output shaft and is in operative coupling with the driving roller; the speed step-up gear is fixedly mounted to the output shaft; and the planet speed-change wheel assembly comprises at least one speed-change gear, each of the at least one speed-change gear comprising a small-diameter toothed portion in mating engagement with the connecting gear and a large-diameter toothed portion in mating engagement with the speed step-up gear; wherein in a low-speed position, the planet speed-change wheel assembly of the speed step-up module is in idling rotation as being set in an orbiting motion so that an input of power is supplied through the driving roller to drive the output shaft in the single direction; and in a high-speed position, the arrestor assembly is activated to stop the orbiting motion of the planet speed-change wheel assembly, so as to have the speed-change gear of the planet speed-change wheel assembly switching from the orbiting motion relative to the speed step-up gear to a spinning motion to step the speed up, wherein the output shaft comprises a shaft expansion section, the driving roller of the transmission module being mounted by means of a one-way shaft assembly to the shaft expansion section, and the driving roller is formed, in a center thereof, with a shaft compartment in which the one-way shaft assembly is arranged, so that the driving roller is operable to drive the output shaft in the single direction and is in idling rotation in an opposite direction; and the planet speed-change wheel assembly comprises two or more than two speed-change gears that are sequentially in engagement with each other and a terminal one of the two or more than two speed-change gears is in mating engagement with the speed step-up gear to improve speed stepping up, characterized in that the driving roller of the transmission module is formed, in a center thereof, with an engaging portion, with which an engaging portion of the connecting gear of the speed step-up module is in mating engagement to have the connecting gear of the speed step-up module and the driving roller moving in synchronization with each other.