Abstract: In hot-dip galvanized zinc-aluminum alloy steel plates, the Fe-Al reaction layer develops between the steel substrate and alloy melt through mutual diffusion mechanism. The Fe/Zn-55%Al-2%Mg-1.6%Si diffusion couple was placed in an alloy melt at 600 ℃ for 1, 2, 3 hours, respectively. The diffusion regularity of the Fe-Al reaction layer in Fe/Zn-55%Al-2%Mg-1.6%Si diffusion couple was studied by using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). The results show that the reaction layer was mainly composed of Al₁₃Fe₄ phase and Al₅Fe₂Zn_0.4 phase. During the diffusion process, Si would hinder the diffusion of Al atoms without affecting the diffusion of Fe atoms. The total thickness of the Fe-Al reaction layer followed a parabolic relationship with diffusion time. The growth thickness toward the steel substrate side accounts for only about 1/5 of the total growth thickness, indicating that the diffusion couple primarily grew toward the liquid alloy coating side.