A method of an optical detecting device for synchronizing an exposure timing sequence of an image detector with a light emitting timing sequence of a reference light source is disclosed. The method includes capturing a continued image set according to a predetermined period, analyzing intensity variation of the continued image set, and adjusting the exposure timing sequence of an image detector according to the intensity variation, so as to synchronize the exposure timing sequence of the image detector with the light emitting timing sequence of the reference light source.