Silver brazing flux, composed of K₂B₄O₇, H₃BO₃, KF and/or KHF₂, was used to remove the oxide film on the surface of 304 stainless steel. The oxide film removal mechanism was studied. The results show that K₂B₄O₇ and KF cannot remove the oxide film on the surface of 304 stainless steel at 700 °C. Under the synergistic action of any two components, only the mixture of K₂B₄O₇ and H₃BO₃ cannot melt at 700 °C. The calculation results of the reaction equilibrium constant (K^θ) and Gibbs free energy (△G_m^θ) show that the main reason for removing oxide film is that F in the brazing flux can replace O in the oxide in the process. XRD results show that the peak pattern at 2θ=18° is changed, indicating that the mechanism of removing oxide film is different when KF is replaced by KHF₂ in silver brazing flux. No. 3 brazing flux with both KF and KHF₂ can effectively remove the oxide film on the surface of the 304 stainless steel plate at 540 °C. The reaction product has the characteristics of amorphous structure.