In recent years, to further improve welding efficiency, SAW with higher heat input has been widely applied. SAW relies on appropriate fluxes to obtain satisfactory weld metal properties. Generally, flux is employed in SAW to stabilize the arc, add alloying elements, and refine WM microstructure. To better control WM composition and offer acceptable mechanical properties, an understanding of element transfer mechanisms between slag and weld pool during welding is necessitated. A series of fused CaF2 based fluxes have been developed to investigate element transfer behaviors under high heat input submerged arc welding. Effects of respective constituent on the transfer of Si, Mn, and O have been thoroughly evaluated. Thermodynamic considerations have been attempted for constraining chemical reactions and mechanisms involved in welding. The transfer of Si and Mn is primarily governed by the slag-metal reactions in the namely ‘hot’ zone while the transfer of O is generally controlled by the chemical reactions in both ‘droplet’ zone and ‘hot’ zone. Our work may provide criteria on the design of CaF2-based fluxes under high heat input.