The film removal mechanism of FB3-F silver brazing flux consisting of H₃BO₃, K₂B₄O₇, and KF with the mass ratio of 7:10:3 was studied. Results show that K₂B₄O₇ or KF cannot individually remove the oxide film on Q235 steel at 700 °C, and KF even accelerates the oxidation rate of steel surface at high temperature. H₃BO₃ can remove the oxide film at 700 °C, but the product possesses an obvious amorphous structure feature and poor fluidity. Furthermore, H₃BO₃ can react with KF at 700 °C, and the reaction product can remove the oxide film. Similarly, K₂B₄O₇ can react with KF, but the product is hard. The mixture solder consisting of H₃BO₃, K₂B₄O₇, and KF can react with the oxide film on surface of Q235 steel plate, and the reaction product presents obvious amorphous feature. The addition of KF transforms the nonreactive H₃BO₃-K₂B₄O₇ binary system into the reactive KF-H₃BO₃-K₂B₄O₇ ternary system at 700 °C. KF shows no corrosivity and promotes the removal of oxide film of steel plates in this ternary system.