This paper aims to study the microstructure and phase composition of hot-dip Al-Si coating on hot-formed steel. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were used to analyze the microstructure and composition of the surface and cross-section of the Al-Si coating; X-ray diffraction (XRD) and electron backscatter diffraction (EBSD) were used to analyze the phase composition and ratio of the Al-Si coating. The surface of the hot-dip Al-Si coating is composed of Al-rich phase, a small amount of Fe-rich phase, and high Si phase with dendritic network distribution; the cross-section of the hot-dip Al-Si coating is composed of inner and outer layers, the inner layer close to the iron matrix is an Fe-Al-Si alloy layer, and the outer layer is an Al-Si layer. Further research shows that the Al-Si layer is composed of Al-rich phase, a small amount of Fe-rich phase, and columnar distributed high Si phase; the high Si phase mainly exists at the interface between the alloy layer and the Al-Si layer and in the Al-Si layer. The phases of Al-rich phase, Fe-rich phase, high Si phase and Fe-Al-Si ternary alloy layer in the hot-dip Al-Si coating are Al, Al13Fe4, Si and Al8Fe2Si respectively. The study of high Si phase in hot-dip Al-Si coating has shown that the high-Si phase distributed at the interface between the alloy layer and the Al-Si layer can effectively hinder the growth of the coating, while the high-Si phase distributed in the Al-Si layer exists in the form of a three-dimensional network framework in space. This three-dimensional network structure as the main frame of the coating can effectively improve the strength and toughness and formability of the coating.