<p>A high-temperature compression experiment was conducted on a 304/Q235 bimetallic composite to study the element diffusion behavior and formation mechanism of the diffusion layer under varying conditions. The experiment was performed at an effective strain of 0.92, a deformation temperature ranging from 1273 to 1373&#xa0;K, and a strain rate between 0.01 and 0.1&#xa0;s^-1. In this paper, the correlation between interface void closure, grain refinement, crystal orientation, and element diffusion behavior was studied, and the formation mechanism and microstructure evolution characteristics were further discussed. Microscopic analysis revealed that as the cavity closure increased, the composite interface underwent point, line, and surface-to-body element diffusion, which led a more compact and integrated bonding at the interface with no clear boundary distinction. The degree of grain refinement differed between the stainless steel (SS) and carbon steel (CS) sides with rising temperature and falling strain rate. The SS-side element diffusion was governed by grain refinement degree, while the CS-side diffusion was governed by crystal orientation consistency. A microstructure with a strong texture ensured the continuity of element diffusion channels, thereby facilitating thorough element diffusion.</p>