The present invention discloses a method of separating, identifying and characterizing cracks in 3D space, which processes as follows to a volumetric image, so as to perform the separation, identification and the characterization of the cracks in the 3D space: 1) preprocessing digital image; 2) statistically analyzing basic information of the digital image: the basic information of the image includes porosity, connectivity of each pore, statistics of pore size, and position, size, orientation and anisotropy of each pore-structure; 3) filtration: removing non-crack structure in the image; 4) smoothening: smoothening and mending the image; 5) thinning: thinning the void structure into a thickness d (d can be any value, but more appropriate to be 2 to 3 voxels generally) in a direction with shortest extension in the 3D space; 6) separation: separating intersected cracks in a crack network by breaking the connections; 7) combination: combining those elongated cracks that are disconnected in the last step, merging tiny structures that are formed during the separation to a nearby large cluster, and restoring cracks to the thickness before thinning, and eventually giving out the characterization of the cracks. In the following expression, the wording “void” is used more, emphasizing the “empty” gap in the image rather than the rock solid. In this patent application, it is mainly for the case where the void appears in a state of crack, not excluding the case where the void appears in a state of small pore.