This study focuses on altering the diffusion combustion characteristics of the self-reflux burner by optimizing the burner nozzle structure. By adjusting the jet angle of each nozzle, enhancing the diffusion effect of fuel and air in the combustion chamber, a low-oxygen combustion atmosphere is formed in the high-temperature zone of the combustion chamber, avoiding local high temperature caused by fuel accumulation and combustion, reducing the combustion rate of nitrogen, and then the thermal NOx emission is reduced. Parallel, inwardly inclined, and outwardly inclined jet angles are studied in simulation, and five working conditions (-10°, -5°, 0°, 5°, and 10°) are calculated, compared, and analyzed. The cooperative coupling mechanism between multiple fields (velocity, concentration, and temperature) is compared and analyzed. We use the diffusion effect to reduce the oxygen concentration in the central axis and achieve the goal of reducing NOx by reducing the maximum combustion temperature. The simulation results show that the maximum and average temperatures of the combustion chamber are reduced by 28.9 K and 15.38 K, respectively, when the jet angle is outwardly tilted by 5°, which reduces the local high temperature; and NOx emissions are in turn reduced by 104.72 ppm. •Low-oxygen combustion technology applies to the self-reflux burner.•We studied the effects of jet angle on the thermal process of diffusion combustion.•We studied the effect of different jet angles on low NOx combustion performance.•We recommend adjusting the jet angle to maximize the self-reflux burner performances.