1. An optical detecting device capable of determining gearshift, comprising: a multi-axis instruction outputting mechanism, an actuating component of the multi-axis instruction outputting mechanism being shifted in reciprocation at a first operating direction to output a corresponding instruction; and an optical detecting module disposed adjacent by the actuating component, an interval between the optical detecting module and the actuating component being varied according to a relative movement between the optical detecting module and the actuating component at a second operating direction, the actuating component being arbitrarily shifted between different gears, the optical detecting module determining the interval and related gearshift according to parameter variation of a reflecting signal actuated by the actuating component and utilizing the reflecting signal to acquire a movement of a feature point on the actuating component along the first operating direction so as to interpret the instruction, wherein the first operating direction is different from the second operating direction.