A detachable speed determining device includes an optical detecting module, an energy generating module and an energy storage module. The optical detecting module is adapted to determine axial rotational displacement of a bike component by detecting a movement of a pattern on the bike component. The energy generating module is adapted to generate energy. The energy storage module is sealed inside a housing of the detachable speed determining device in a water-resistance manner and electrically connected with the optical detecting module and the energy generating module. The optical detecting module is powered by the energy storage module, and the energy storage module is charged by the energy generating module. The detachable speed determining device further includes a bolted joint and a quick releasing handle, the bolted joint passes through an axle of the bike component, and the quick releasing handle is foldably disposed on the bolted joint.