<p>The Kelvin–Helmholtz (KH) instability in the gravitational field with various density stratifications is simulated using a two-component discrete Boltzmann method. The influence of Atwood numbers ranging from negative to positive is investigated through key aspects, including concentration gradients, mixing degree, amplitude, and vorticity dynamics. The results show that concentration fraction gradients and vorticity increase with higher Atwood numbers. Conversely, the mixing degree and amplitude initially decrease but later increase as the Atwood number rises. Furthermore, a detailed analysis of the vorticity equation terms reveals that the Atwood number significantly affects vorticity evolution. Interestingly, when the Atwood number is zero, the temporal accumulation of these terms is minimal. Physically, the KH instability enhances the growth of the interface and mixing degree, while diffusion broadens the transition layer. Additionally, the Rayleigh–Taylor (RT) instability extends the perturbed interface vertically and promotes the mixing of the two media if the upper medium is heavier than the lower one; otherwise, the RT stabilization suppresses these effects.</p>