1. An automatic range finding method capable of measuring a distance between a stereo camera and a reference plane, the automatic range finding method comprising: acquiring a disparity-map video by the stereo camera facing the reference plane; analyzing the disparity-map video to generate a depth histogram; selecting a pixel group having an amount greater than a threshold from the depth histogram; calculating the distance between the stereo camera and the reference plane by weight transformation of the pixel group; acquiring at least one parameter of an object according to the distance between the stereo camera and the reference plane; utilizing field of view information of the stereo camera and the distance between the stereo camera and the reference plane to decide tracing computing parameters M and N, wherein M and N are positive integers; dividing the disparity-map video into a plurality of coarse candidate zones wherein each coarse candidate zone has a length of M pixels and a width of N pixels; determining whether to set at least one coarse selected zone from the plurality of coarse candidate zones; enlarging the coarse selected zone to acquire a coarse selected enlarged zone; dividing the coarse selected enlarged zone into a plurality of fine candidate zones wherein each fine candidate zone has a length of m pixels and a width of n pixels, wherein m and n are positive integers; determining at least one fine selected zone from the plurality of fine candidate zones; and acquiring position of a mass center of the at least one fine selected zone.