Abstract: The high temperature flame in the reformer of non-catalytic partial oxidation (NC-POX) of methane under pure oxygen atmosphere will affect the safe and stable operation of the actual industrial plant. Adding diluent to dilute pure oxygen in the reaction system can improve the temperature distribution and flame structure of the reformer. The volume ratio of O₂ to CH₄ 0.69 was typically used when NC-POX reformers were used industrially. Numerical modelling was conducted using ANSYS Fluent 16.1 commercial simulation software, with a realizable k-ε turbulence model considered for Reynolds-averaged Navier-Stokes (RANS) model. The Eddy Dissipation Concept (EDC) model coupled with GRI3.0 chemical reaction mechanism was suitable for simulating turbulent combustion and chemical reaction processes in the reformer. This study investigated the effect of three diluent (N₂, CO₂, H₂O) on the distribution of temperature, radicals (·OH、·CH₂O) and flame reaction zone. The results indicated that effects of N₂ and H₂O dilution on the temperature, radicals and flame reaction zone distribution were basically indistinguishable, and the effect of CO₂ dilution was the most significant. The increasing of CO₂ content decreased the peak temperature by about 50 K and peak values of ·OH and ·CH₂O radicals decreased by about 33.0% and 24.5%, respectively. In addition, three effects of CO₂ dilution on the temperature and radicals distribution in the reformer were in the order of dilution effect > thermal effect > chemical effect.