CO2 dilution is a well-recognized effective strategy for combustion control. Against the backdrop of global energy structure transformation and carbon neutrality goals, developing efficient, clean, and low-carbon combustion technologies is crucial. However, the high proportion of CO2 in flue gas recirculation or the use of low-calorific-value fuels rich in CO2 can complicate the temperature field and NOx generation paths in combustion chambers, especially under lean fuel conditions. This study aimed to explore the detailed dilution effect of CO2 on the combustion of CH4 and H2, and thus numerically analyzed the combustion characteristics of jet diffusion flames. The numerical simulation was conducted under specific conditions: CO2 mixing ratio ranging from 0% to 40%, ambient temperature of 300 K, pressure of 101 325 Pa, and equivalence ratios between 0.4 and 0.9. A new triangular nozzle burner was adopted, along with the standard k-ε turbulence model and non-premixed combustion model for calculations, and grid independence verification was performed to ensure result reliability. The results showed that CO2 blending reduced the peak flame temperature, flame height, and average furnace temperature (from higher values to 1344 K at 40% CO2 mixing ratio). It inhibited the forward reactions of CO combining with O and OH, reducing O2 consumption, and achieved a maximum NOx reduction rate of 95.5% compared to pure methane combustion. In contrast, H2 enrichment increased the flame temperature, accelerated the combustion reaction, and formed a more compact flame structure. When CO2 was added to the CH4/H2 mixture, it still effectively reduced NOx emissions, though the reduction effect weakened slightly with higher CO2 blending ratios. So CO2 dilution was an effective method to achieve ultra-low NOx emissions in industrial boilers, especially suitable for hydrogen-rich natural gas combustion. Controlling CO2 blending below 30% balanced NOx reduction and combustion stability. The CH4/H2/CO2 ternary mixing strategy enabled coordinated regulation of the temperature field and pollutant emissions, providing a technical pathway for the industrial application of hydrogen-rich fuels.