Abstract: The thermal hydrogen atom source is crucial in material processing and semiconductor technology. Its core principle is to generate atomic hydrogen (H) through the catalytic cracking of hydrogen molecules (H₂) at high temperatures. Considering that the continuum hypothesis is no longer applicable under the transitional flow or molecular flow regime in the cracking chamber, this study constructs a catalytic cracking wall reaction model on the OpenFOAM platform. By integrating the Direct Simulation Monte Carlo (DSMC) method with the microscopic reaction probability formula, we reveal the cracking mechanism of hydrogen molecules on the surface of the hot tungsten filament and the characteristics of the flow-field distribution. The simulation results show that at a filament temperature of 2500 K and hydrogen flow rate of 0.176 sccm, the cracking efficiency reaches 28%, which drops to about 12% when the flow rate increases to 1.056 sccm. The cracking efficiency grows exponentially with higher filament temperature but decreases significantly with increased flow rate. Cercignani–Lampis–Lord (CLL) model analysis reveals it is mainly regulated by the tangential accommodation coefficient (negligible impact from the normal one). H₂ and H atoms differ notably in velocity and density distribution, and the inlet slit effectively suppresses H atom backflow.These results confirm the DSMC method applicability and the model effectiveness, providing key references for optimizing hydrogen atom source cracking processes and structure.