Abstract: To investigate the influence of the aspect ratio on the flow field and acoustic field of a rectangular nozzle, study has been conducted using DES/FW–H hybrid algorithm on two different aspect ratios of rectangular supersonic fully expanded jet nozzles. The influence of the aspect ratio on flow dynamics and noise of the jet has been analyzed. Firstly, multiple flow field variables were compared and analyzed to verify the feasibility of numerical simulation methods. Differences were observed in the pressure changes on the inner wall surface near the outlet for different aspect ratios, with the larger aspect ratio exhibiting faster pressure changes. Also, it was found that the velocity decreases more rapidly with distance from the outlet on the short axis of the lip, while on the long axis of the lip, the velocity decreases slowly. Next, relating the aspect ratio of the nozzle exit to the jet noise field, and comparing existing noise experimental data with computed data, it was found that in all angles, the maximum difference between the experimental and computed total sound pressure level was 2.6 dB (AR = 3) and 4 dB (AR = 1.5). Increasing the aspect ratio could reduce the total sound pressure level upstream. Furthermore, changes in the shear layer thickness under different aspect ratios were analyzed, and the impact of these changes on the jet noise was studied. The results reveal that increasing the aspect ratio would increase the shear layer thickness and shift the rapid expansion location of the shear layer and high-frequency noise sources upstream. Finally, the phase velocities of specific frequency noise at the lip of the exit were compared for different aspect ratios, revealing a significant reduction in the phase velocity along the long axis of the exit lip with increasing aspect ratio, which affects the angle of the near-field noise radiation.