An optical detecting device includes a multi-axis instruction outputting mechanism and an optical detecting module, and shift of the multi-axis instruction outputting mechanism is determined accordingly. An actuating component of the multi-axis instruction outputting mechanism is moved in reciprocation at a first operating direction to output an instruction. The optical detecting module is detachably disposed to the actuating component. An interval between the optical detecting module and the actuating component is varied according to a relative movement between the optical detecting module and the actuating component at a second operating direction, and the actuating component can be shifted between different gears. The optical detecting module determines the interval and gearshift according to variation of a reflecting signal actuated by the actuating component, and utilizes the reflecting signal to acquire a movement of a feature point detachably disposed on the actuating component along the first operating direction to interpret the instruction.