1. A focus identification method, the method being executed by at least one processor of a computing device, the method comprising: acquiring discrete image data from a plurality of images of an object, wherein the plurality of images are captured by a charge coupled device (CCD) of a measurement machine every a predetermined time period when the CCD moves along a Z-axis of the measurement machine at a fixed speed, and the discrete image data comprises definitions of the plurality of images and Z-coordinates of the CCD when capturing the plurality of images; computing maxima and minima of the discrete image data; establishing intervals of the discrete image data according to the maxima and minima; selecting an interval from the established intervals; fitting a curve according to the discrete image data in the selected interval, and computing a mathematical expression of the fitted curve; and computing a peak value of the fitted curve as one of focuses of the object according to the mathematical expression, and storing the mathematical expression and the peak value into a storage device of the computing device upon condition that a goodness of fit of the fitted curve meets a predetermined criterion; computing the maxima and the minima of the discrete image data by a formula as follows: 
 X={X[ 0], X[ 1],. .. , X[n− 1]}, wherein X[ 0], X[ 1],. .. , X[n− 1] are the discrete image data; The first derivative of the above forum is: 
 dX=diff ( x )={ X[ 1]− X[ 0],. .. , X[n− 1]− X[n− 2]}; and The second derivative of the above forum is: 
 d 2 X=diff ( diff ( x ))={ dX[ 1]− dX[ 0],. .. , dX[n− 2]− dX[n− 3]}; The maxima of the discrete image data satisfy diff(sign(diff(X)))=−2, and the minima of the discrete image data satisfy diff(sign(diff(x)))=2 wherein sign(r) is a sign function which returns a value 1 when r>0, returns a value −1 when r<0, and returns a value 0 when r=0.