This paper studies the impact dynamics process of the CDH-125 pneumatic down-the-hole （DTH） hammer， accurately analyzing the force state during the motion of the valueless distribution piston. Subsequently， based on the finite difference method， the thermodynamic and dynamic differential equations of the DTH hammer during its working process are iteratively solved to obtain the dynamic working characteristics of the DTH hammer piston and air chambers. These characteristics include the relative relationship between piston displacement and velocity， and the pressure and temperature exchange relationships in the front and rear air chambers. The results obtained from the finite difference method were compared and validated using the multiphysics simulation software SimulationX， showing a maximum speed discrepancy of 3% for the piston stroke， 19% for the return stroke， and 5% for the working frequency. Based on these results， further calculations and analyses of the air viscous friction force between the piston and the inner cylinder were conducted. The results indicate that， compared to the ideal situation with non-viscous fluid， the viscous friction force reduces the working frequency of the DTH hammer by 0.15% per working cycle and increases the maximum energy consumption of the piston stroke by 0.05~1.34J per cycle. The impact of viscous friction on the dynamic performance of the DTH hammer cannot be ignored.