Abstract: The effect of argon-to-hydrogen mole ratio on the discharging behavior of argon-hydrogen plasma was simulated and studied on the assumption that argon-hydrogen plasma is in a local thermodynamic equilibrium. The kinetic theory of ideal gases and the classical Chapman-Enskog method were employed in the study. The plasma thermodynamic and transport parameters consistent with the actual condition of the DC arc plasma jet method were firstly found, and secondary development was made on FLUENT software platform. Equations, like current continuum and Ampere's law, and source items, like Lorentz force and Joule heat, which are associated with electromagnetic fields were also taken into account. The results show that when the gas pressure and operating current are 8 kPa and 150 A, respectively, and the argon-to-hydrogen mole ratio changes from 3:1 to 1:3, the maximum flow rate of plasma increases from 829 to 1127 m·s^-1, the maximum temperature falls from 20600 K to 16800 K, and the heating capacity of the DC arc improves while the substrate surface temperature uniformity deteriorates. Under the other conditions being unchanged, when the argon-to-hydrogen ratio is 1:2, a relatively uniform and proper substrate surface temperature can be obtained for the growth of diamond films.