Abstract: It is of great significance to obtain accurate mineral composition of rock and the meso-structure of minerals to study the macroscopic mechanical properties of rock from the meso-level. CT scanning technology is a powerful tool for studying the meso-structure of rocks. In this paper,we use a high-resolution 3D CT scanning system to obtain CT images of granite. We determine reasonable segmentation thresholds by considering the results of X-ray diffraction test. And then,we realize the three-valued analysis of CT grayscale images of granite by threshold segmentation method. Finally,we obtain the main mineral content and meso-structure characteristics of granite. The results show that the main minerals of granite,i.e.,quartz,feldspar,and mica can be identified and quantitatively determined by using reasonable segmentation thresholds. Based on the 3D reconstruction model,we can obtain the shape,the particle size,and the spatial distribution characteristics of different minerals. The method proposed in this paper can provide some references for testing the rock mineral composition quantitatively and understanding their meso-structures.