The invention discloses a welding flux for X100 pipe line steel submerged arc-welding and a preparation method thereof. The welding flux comprises, in percentage by weight, main components including 5-13% of SiO2, 26-40% of MgO, 15-24% of Al2O3, 18-25% of CaF2, 1-4% of CaO, 4-8% of MnO, 1-2% of Na2O, 1-2% of K2O and 1.0-6.0% of TiO2, and a bonder accounting for 15-20wt% of the total weight. The preparation method comprises the steps of: uniformly mixing the main components according to proportions; then adding the bonder for pelleting; and baking at low temperature, sintering at high temperature and sifting. When the welding flux is matched with a special welding wire CHW-SG10, a soldered seam has excellent performances, such as high strength, high toughness, better corrosion resistance, lower rigidity and the like approaching to those of an X100-level steel pipe body, and the welding flux can be used for carrying out single or multiple-wire welding and internal and external high-speed welding; and a multiple-wire large-line energy welding speed can reach 120 m / h, and the soldered seam performance and air holes, slag inclusions, cracks, appearance edge, undercut and the like of the soldered seam all meet the requirements of the technical standard of the X100 pipe line steel submerged arc-welding.