An optical detecting device includes a light source, an optical detecting component, a package structure and a light tight component. The light source outputs a sampling signal to project onto an external object. The optical detecting component is disposed by the light source and has an interval relative to the light source. The optical detecting component receives the sampling signal reflected from the external object. The package structure covers the optical detecting component and the light source, and includes an illuminating surface unit and an incident surface unit respectively corresponding to the light source and the optical detecting component, and further includes an isolation component disposed between the illuminating surface unit and the emerging surface unit. The light tight component is disposed on the isolation component to obstruct transmission path of the sampling signal from the illuminating surface unit to the incident surface without reflection by the external object.