The speed of foreign high-speed electroslag surfacing with band electrode has reached more than 40 cm/min and the welding process performance is good, while the domestic supporting flux for high-speed electroslag surfacing with band electrode is prone to defects such as undercut, poor weld bead forming and unstable product quality. This kind of welding material mainly relies on import, which restricts the development of China's manufacturing industry to a certain extent. A kind of sintered flux CHF25B for strip electroslag surfacing of austenitic stainless steel is developed. The slag system is CaF,2,-Al,2,O,3,-SiO,2, and its alkalinity is from 3.8 to 4.0. In the welding test of 15CrMoR steel with this flux combined with welding strips EQ309LMo and EQ316L, the welding speed can reach 40cm/min, and the electroslag process is stable, the welding spatter is small, the slag removal is easy, the weld bead surface is flat and smooth, and the lap joint between weld beads is well fused. The content of impurity elements of the surfacing metal such as S and P is low and burning loss of alloy elements is less. The chemical composition and performance of surfacing metal meet the requirements of NB/T 47018.5-2017 and design technical conditions.