<p>To ensure that welding joints in stainless-steel composite materials exhibit favorable corrosion resistance. In this study, advanced material analysis techniques, such as electron backscatter diffraction, energy dispersive spectroscopy, and x-ray diffraction, were employed to investigate the microstructure, phase composition, and elemental distribution at the interface between transition and cladding layer welds in a S32304/Q390C composite material. Molecular dynamics simulations were conducted to model atomic diffusion behavior at the interface on the basis of theoretical principles. Key diffusion parameters, including mean square displacement, diffusion activation energy, and diffusion coefficients of the interface atoms, were calculated and analyzed. Results indicate that during welding, Fe and Cr atoms predominantly diffused during atomic diffusion at the interface. The dominant diffusion mechanisms were identified as ring-exchange diffusion, which is a subset of displacement diffusion. The geometrically necessary dislocation density at the interface was relatively low, the distribution of Fe, Cr, Ni, and other components was uniform, and no evidence of harmful phase precipitation was detected at the interface. The diffusion coefficients of Fe and Cr atoms on both sides of the interface increased with the welding line energy of the cladding layer weld, but their diffusion distances remained basically unchanged. At the same temperature, the diffusion coefficient of Cr atoms was an order of magnitude larger than that of the Fe atoms. The microstructure of the transition layer weld consisted of austenite and skeletal ferrite, whereas the microstructure of the cladding weld consisted of austenite and ferrite. Columnar austenite morphology was the predominant morphological form.</p>