1. A method of synchronization adjustment applied to an optical detecting device for synchronizing an exposure timing sequence of an image detector with a light emitting timing sequence of an indicating light source, the method comprising: acquiring a continued image set; analyzing intensity of each image of the continued image set; and adjusting the exposure timing sequence according to intensity ratio of at least two images and a duty cycle of the image detector; wherein an exposure frequency of the image detector is substantially greater than a flickering frequency of the indicating light source.