A brake sensor apparatus is provided, including a base unit, a movement unit, a spring, a first buckle unit and a second buckle unit. The movement unit is partially confined inside the base unit so that the movement of the movement unit is restricted to a short distance and along the direction of the base unit. The spring is to allow the retracted movement unit to be moved to the original position. The first buckle unit and the second buckle unit are engaged respectively to the outer wall of base unit and movement unit, and can be hanged and buckled to the positions of brake wire of the brake. In this manner, when the bicycle brakes, the movement unit is retracted into the base unit so as to trigger the trigger switch to connect the connected circuit.