Abstract: The effects of different grain size and trace element content on the corrosion behavior of 310S stainless steel in molten chloride salt NaCl-KCl-MgCl₂ at 800 ℃ in an inert environment were studied. The corrosion rate of the alloy was measured by the loss-in-weight method and the corrosion kinetic curve was drawn, and the surface and cross-section morphologies of the corrosion products were characterized by XRD, SEM and EDS. The results show that the corrosion rate of the alloy decreases significantly with the increase of corrosion time. After 500 h of corrosion, the corrosion rates of the alloy with grain size grade 5 and grade 7 are 622.80 μm/a and 952.26 μm/a, respectively. Cr and Si of 310S steel are selectively dissolved, and react with impurities and molten salt to form corrosion products such as Mg O,CrCl₂, MgCr₂O₄ and MgSiO₃. Corrosion is more likely to occur at the intersection of grain boundaries, and the corrosion rate of the alloy is mainly controlled by the diffusion rate of Cr in the matrix. Grain boundaries are the fast diffusion channels of Cr. The steel with grain grade 7 has smaller grain size, larger grain boundary area, and the Cr diffusion rate is faster, so the corrosion rate is bigger.