To study the effect of the density current driven by temperature difference on the flow structure and mixing characteristics at the confluence of the Yangtze River and Poyang Lake, a three-dimensional hydrodynamic numerical model of the confluence channel of the Yangtze River and Poyang Lake was constructed based on field monitoring data, and the hydrodynamics and mixing processes at the confluence under conditions of different density differences were simulated and analyzed. The results indicate that the shear layer at the confluence of the Yangtze River and Poyang Lake tilts toward the side with a higher density, and a larger density difference between the two branch flows indicates a more obvious tilt of the shear layer; when the density of the Yangtze River water body is lower than that of Poyang Lake, a clockwise single-helix secondary flow is formed at the confluence, and conversely, a pair of counter-rotating double-helix secondary flows are formed; the density difference effect enhances the intrusion effect of the low-density water body into the high-density water body near the water surface, and when the density of the Yangtze River water body is higher than that of Poyang Lake, it has a promoting effect on mixing, and conversely,it suppresses mixing.