Abstract: As a plastic fluid, the morphological features of internal structure in an unclassified tailing paste closely relate to its flow properties. Studies on the evolution characteristics of internal structure during the sheafing flow process have extremely important significance for analyzing flow behavior in pipeline transportation and paste preparation. In this paper, scanning electron microscopy was used to investigate the meso-structure images of the paste at different mixing time. The fractal dimensions of these meso-structure images were estimated by the image processing technology and fractal theory. A structure coefficient A was put forward to explain the morphological features of meso-structure, and a time-evolution mathematical model of meso-structure was built in the mixing process. Some related experiments were done for a lead-zinc tailing paste. It was found that the structure coefficient A flattened out gradually after a steep fall with mixing time, and finally reached an equilibrium state. The failure coefficient and restitution coefficient in the evolution model were obtained to be 0.171 and 0.491 respectively by fitting experimental data.