Abstract: In this article, laser welding experiment of 1.5 mm thick Al-Si coated hot formed steel was carried out with 45 steel foil as interlayer by CWX3000 fiber laser. The effect of the carbon content of fusion zone on the microstructure and mechanical properties of the welded joint was studied. The results showed that after adding foil, the carbon content of fusion zone increased, the austenite phase zone was expanded, the δ-ferrite fraction of fusion zone decreased from 17.3% to 4.5%, the average hardness of fusion zone increased from 425HV to 557HV, the tensile strength of welded joint increased from 980MPa to 1280MPa, and the cupping value increased from 1.7mm to 3.2mm. Tensile strength and formability are both improved.