Abstract: Matrix swelling effect will cause the change of microstructure of coal reservoir and its permeability, which is the key factor affecting the engineering effect of CO2-ECBM technology. The Sihe and Yuwu collieries are taken as research objects. Firstly, visualization reconstruction of coal reservoir is realized. Secondly, the evolution of the pore/fracture structures under different swelling contents is discussed. Then, the influence of matrix phase with different swelling contents on permeability is discussed. Finally, the mechanism of swelling effect during the CO2-ECBM process is further discussed. The results show that the intra-matrix pores and matrix-edge fractures are the focus of this study, and the contacting area between matrix and pore/fracture is the core area of matrix swelling. The number of matrix particles decreases with the increase of size, and the distribution of which is isolated with small size and interconnected with large size. The swelling effect of matrix particles with larger size has a great influence on the pore/fracture structures. The number of connected pores/fractures is limited and only interconnected in a certain direction. With the increase of matrix swelling content, the number, porosity, width, fractal dimension, surface area and volume of pores/fractures decrease, and their negative contribution to absolute permeability increases from 0.368% to 0.633% and 0.868%-1.404%, respectively. With the increase of swelling content, the number of intra-matrix pores gradually decreases and the pore radius becomes shorter during the CO2-ECBM process. The matrix continuously expands to the connected fractures, and the width of connected fractures gradually shorten. Under the influence of matrix swelling, the bending degree of fluid flow increases gradually, so the resistance of fluid migration increases and the permeability gradually decreases. This study shows that the matrix swelling effect is the key factor affecting CBM recovery, and the application of this effect in CO2-ECBM process can be discussed.