A device for tightening and loosening a bicycle's brake wire is provided, which is installed between a handbrake operating device and a handbrake assembly of the bicycle. The device is mainly composed by a knob structure, a seat structure, and a tubular element. The knob structure and seat structure are configured such that their total length changes as the knob structure is twisted, which in turn tightens or loosens the brake wire of the bicycle. The gap between the handbrake assembly's brake pads therefore could be narrowed for the handbrake's normal operation, or loosened for dismounting the bicycle's wheel quickly and conveniently.