Abstract: The mechanism of dephosphorization and desulphurization in melting pool during submerged arc welding has been investigated by experiments and theoretical analyses. It is found that the dephosphorization and desultphurization are mainly done by the metallurgical reactions of forming phosphates and sulphates respectively, and that there is a limit for the dephosphorization and desulphurization in the melting pool. The limit value depends on the basicity of flux as well as the contents of phosphorus and sulphur in the wire and the base metal. For high basicity fluxes of B₁≥2.5, the limit value of sulphur in weld metal can be lower than that in the wire or can be close to that in the flux according to the content of sulphur in wire or flux, and the limit value of phosphorus is about 0.010%±0.002.