The invention discloses a light-weight speed changing device and a bicycle adopting the light-weight speed changing device. The speed changing device comprises a differential mechanism case, a gear shifting shaft, a gear shifting chain wheel group and a gear shifting rod, wherein the axis of the gear shifting shaft is parallel to the axis of the differential mechanism case, the gear shifting chain wheel group comprises a gear shifting driving chain wheel and a gear shifting driven chain wheel, the gear shifting driving chain wheel is arranged on the gear shifting shaft in a sleeving way, the gear shifting driven chain wheel is arranged and connected on the differential mechanism case in a sleeving way, and is connected with the gear shifting driving chain wheel, and an annular convex block is arranged on the circumferential surface of the gear shifting rod. The annular convex block of the gear shifting rod moves to a connecting part of the gear shifting driving chain wheel and the gear shifting shaft, so that the gear shifting driving chain wheel and the gear shifting shaft are locked, input force is provided for the gear shifting shaft, the gear shifting driving chain wheel rotates along with the rotation of the gear shifting shaft, different bicycle speeds are realized through different transmission ratios of the gear shifting chain wheel group, the structure is light and convenient, the influence on the running speed of the bicycle is smaller, in addition, the occupied space is small, the small size is realized, the riding is easy, on the other hand, the gear shifting rod can be respectively moved in the motionless state and in the operation state of the bicycle for realizing the large-speed-ratio gear shifting, the fast starting is convenient to realize, and more time and more labor are saved.