Abstract: According to the characteristics of the transverse submerged arc welding process of 9Ni steel for Liquefied Natural Gas（ LNG） storage tank,CaF₂-MgO-SiO₂-Al₂O₃ fluorine alkaline slag system were determined based on the forming mechanism and the properties of Ni Cr Mo-4 welding material. And then,the formula was designed and the experiments were carried out. The results shows that the formula has good welding usability,the properties of the deposited metal are stable,the cleanliness of the welds is good and the shape,quantity,size and distribution of the inclusions are beneficial to the mechanical properties of the welds.