1. A rapid autofocus method for a stereo microscope, comprising steps of: {circle around (1)} acquiring a stereo microscopic calibration image sequence having M stereo microscopic images, wherein: the M stereo microscopic images in the stereo microscopic calibration image sequence are orderly arranged from vague to clear and then from clear to vague, and contain the same target object; an i th stereo microscopic image in the stereo microscopic calibration image sequence is denoted as I i , wherein M≧50 and 1≦i≦M; each of the stereo microscopic images consists of a left view image and a right view image with the same resolution; {circle around (2)} calculating a disparity of each of the stereo microscopic images in the stereo microscopic calibration image sequence, wherein the disparity of I i is denoted as d i ; {circle around (3)} extracting a clear stereo microscopic image sequence out of the stereo microscopic calibration image sequence, comprising steps of: {circle around (3)}-1, calculating a value of a Tenengrad function of a left view in each of the stereo microscopic images in the stereo microscopic calibration image sequence with a Tenengrad definition operator; and {circle around (3)}-2, finding out a largest value among the M values of the Tenengrad function, denoted as F Tm L ; according to a range of a depth of field at a chosen magnification, picking out m stereo microscopic images before the stereo microscopic image corresponding to F Tm L , and also m stereo microscopic images after the stereo microscopic image corresponding to F Tm L ; and then, constructing the clear stereo microscopic image sequence orderly with the m stereo microscopic images before the stereo microscopic image corresponding to F Tm L , the stereo microscopic image corresponding to F Tm L and the m stereo microscopic images after the stereo microscopic image corresponding to F Tm L ; wherein: a value of m is related to the magnification of an objective lens of the stereo microscope as: m = n 2 × NA × Δ ⁢ ⁢ h ⁢ ( λ + e β ) , wherein: n represents a refractive index of a medium between an object provided on a vertical elevator and the objective lens of the stereo microscope; λ represents a light wavelength; e represents a minimal physical distance distinguishable between two neighboring light sensors on an image plane of the stereo microscope; β represents the magnification of the objective lens of the stereo microscope; NA represents a numerical aperture of the objective lens of the stereo microscope; NA=n×sin θ; θ represents an angle formed by a refraction light farthest from a central optical axis of the objective lens of the stereo microscope; Δh represents a distance by which the vertical elevator moves each time, having a unit of μm; 2≦Δh≦12; {circle around (4)} finding out the disparity smallest in value and the disparity largest in value from the disparities of all the stereo microscopic images in the clear stereo microscopic image sequence, respectively denoted as d Cmin and d Cmax , wherein an interval [d Cmin , d Cmax ] represents a disparity range of clear stereo microscopic images at the chosen magnification; {circle around (5)} at the chosen magnification, arbitrarily acquiring a first stereo microscopic image after imaging by the stereo microscope, denoted as I test ; calculating the disparity of I test , denoted as d test ; and then realizing an autofocus of a target object in I test according to d test and [d Cmin , d Cmax ], comprising steps of: {circle around (5)}-1, judging whether |d test −d mc |<T or not; if yes, determining that I test is the clearest image and realizing the autofocus of the target object in I test ; if no, executing a step {circle around (5)}-2; wherein: “| |” is a symbol for counting an absolute value; d mc represents the disparity corresponding to the clearest image in [d Cmin , d Cmax ]; d mc = d C ⁢ ⁢ min + d C ⁢ ⁢ max 2 ; T represents a designated judgment threshold; T∈(0,1]; {circle around (5)}-2, supposing that the vertical elevator moves by Δh μm each time, calculating times at which the vertical elevator needs to move for focusing the target object in I test to a clearest position, denoted as step, wherein step = d test - d mc d perstep ; determining a direction towards which the vertical elevator moves for focusing the target object in I test to the clearest position according to step ; if step is smaller than 0, determining the direction to be upward; if step is larger than 0, determining the direction to be downward; wherein: 2≦Δh≦20; d perstep represents an absolute value of a difference between the disparity of the stereo microscopic images acquired before the vertical elevator moves upward or downward at one time and the disparity of the stereo microscopic images acquired thereafter; {circle around (5)}-3, moving the vertical elevator at the times, step, and toward the direction for focusing the target object in I test to the clearest position; and {circle around (5)}-4 after finishing moving the vertical elevator, acquiring a second stereo microscopic image after imaging by the stereo microscope, denoted as I test ′; calculating the disparity of I test ′, denoted as d test ′ ; and then, let I test =I test ′ and let d test =d test ′, returning to the step {circle around (5)}-4 and executing an autofocus iteration, wherein “=” in I test =I test ′ and d test =d test ′ is an assignment symbol.