An optical detecting device is utilized to determine a relative position of a reference object or a light source according to an optical reflecting signal reflected from the reference object via an optical detecting signal emitted by the light source. The optical detecting device includes a light penetrating component, at least one light tight structure and an optical detecting component. A focal length of the light penetrating component is greater than a predetermined distance. The light tight structure is located on a region correlative to the light penetrating component. The optical detecting component is disposed by the light penetrating component and spaced from the light penetrating component by the predetermined distance. The optical reflecting signal is projected onto the optical detecting component through the light penetrating component to form a characteristic image via the light tight structure, and the characteristic image can be used to determine the relative position.