The problems of long flame and high pollution emissions in low-power burners are of wide concern in small heating devices. To solve this problem, a small diesel burner with self-circulation flue gas was designed herein. In order to obtain a deeper scientific understanding of the flame and emission characteristics of the burner with different spray cone angles, a numerical calculation method was used to investigate them. Reasonable flow, heat transfer, and combustion models were selected, and periodic boundary conditions were used to verify the feasibility of the numerical model. The results indicate that the flame length increases with increasing spray cone angle, and then the flame length basically stabilizes at 410 mm. The maximum flame temperature decreases slightly with increasing spray cone angle. Besides this, the NO emission of this small flue gas self-circulation burner decreases with increasing spray cone angle and is as low as 10 ppm at an 80? spray cone angle. In addition, the influence mechanism of the spray cone angle on the flue gas self-circulation ratio was analyzed from the physical aspect of the spray area and the chemical aspect of combustion. This study is of great significance to research on the flame morphology of small flue gas self-circulation burners and the selection of different spray cone angles.