Abstract: Chlorine dioxide (ClO₂) is effective on gel-breaking of reservoir fracturing for coalbed methane wells, as well as on weakening the affinity for methane and increasing the permeability of coal reservoir. To investigate the mechanism of ClO₂ modifying the physical properties of coal reservoirs is of great significance in the recognition of using high-viscosity fracturing fluid instead of commonly-used active water for deep coal reservoir modification. Coal samples with different metamorphic grades were selected and further examined by Raman spectra and mercury injection, aiming to compare the modification characteristics of porous and chemical structures of coal before and after the treatment of ClO₂. The analytical results show that:(1) The porosity and pore volume of coal samples were enhanced to varying degrees, while the specific surface area decreased. (2) The branching degree of aliphatic chains and condensational degree of aromatic rings of the post-modified coals both saw a decrease. Additionally, the defect of structural units increased, while the total amount of aromatic carbon reduced. Thus, the mechanism of ClO₂ modifying the physical properties of coal reservoir lie in the corresponding changes of porous and molecular structures of post-treated coals.