There is provided an operating method of a distance measurement system including the steps of: successively capturing image frames with an image sensor; controlling a sampling parameter to converge an average brightness value of an object image in the image frames to be within a predetermined range; calculating a plurality of first object sizes of a converged object image in a converged image frame and calculating a first average value of the first object sizes; calculating a second average value of the first average values corresponding to a plurality of converged image frames; and comparing the second average value with a lookup table to determine an object depth.