Abstract: Density variation is a key factor affecting the mixing characteristics and flow stability of variable-density jets. For variable-density jets where three types of gases (helium, air and carbon dioxide) are injected into ambient air with density ratios of 0.14, 1.00 and 1.52 respectively, large eddy simulation (LES) is adopted to investigate the evolution of mean flow fields, vortex structures and coherent structures under identical inlet momentum flux. The results reveal that the velocity and concentration of the helium jet decay rapidly, followed by the air jet, while the carbon dioxide jet exhibits slow decay of velocity and concentration. Vortex structures in the flow field are identified using specific criteria. It is found that the vortex structures of the helium jet become unstable and break up earlier, with larger vortex scales and faster radial diffusion. In contrast, the vortex structures of the carbon dioxide jet remain stable, break up later, feature smaller vortex scales and show concentrated radial distribution, and the vortex structures of the air jet lie between the two cases. On this basis, dynamic mode decomposition (DMD) and proper orthogonal decomposition (POD) are performed on the pressure field of variable-density jets. The results indicate that the coherent structures of the carbon dioxide jet are stable, whereas those of the helium jet are unstable, which makes its vortex structures more prone to instability and breakup. Furthermore, the wave packet structures in the helium jet account for its high entrainment rate.