Abstract: In Tokamak exhaust processing system, steam methane reforming reaction is expected to convert deuterium and tritium in the gaseous compounds to elementary gas, then the deuterium and tritium can be reclaimed. The method of Gibbs free energy minimization was employed to analyze the thermodynamic balance of steam methane reforming reaction. The effect of many factors, such as reaction temperature, reactants ratio, pressure, O₂, CO₂, H₂ and CO, was investigated. The appropriate reaction condition is as follows: The temperature is between 650 ℃ and 700 ℃, the pressure is 1×10⁵ Pa, and the ratio of water to methane is 1.5-2.0. Moreover, the presence of O₂ or CO₂ in reactants is favorable to lessen the yield of solid carbon and increase the conversion rate of hydrogen isotope. The presence of H₂ does not affect the thermodynamic balance obviously, while CO increases solid carbon yield, which is a disadvantage to the reaction, so it needs to be wiped off before reaction.