The present disclosure relates to a Completely Automated Public Turing Test to tell Computers and Humans Apart (CAPTCHA) image authentication method and system. The CAPTCHA image authentication method comprises the steps of: collecting a plurality of first objects; defining a plurality of variables so as to be used as basis for classifying and dividing the plural first objects into M groups accordingly while allowing each group in the M groups to correspond to at least one variable selected from the plural variables; selecting at least one group from the M groups while further grading and dividing the first objects in the selected group into subgroups of N grades based upon a standard unit of the variable corresponding to the selected group; sorting and storing the subgroups of N grades; and selecting a plurality of authentication objects from the subgroups of N grades to be used in an authentication process.