To investigate the effect of the preheating temperature of the substrate, the preheating temperature of Q235 steel was varied, and then, it was observed what changes it produced on the organization and properties of iron-based amorphous composite coatings prepared by laser melting and cladding. XRD, microanalysis, hardness testing, friction and wear, and electrochemical corrosion experiments were used for comprehensive evaluation. The results show that the preheating temperature significantly affects the coating amorphous phase content, crack formation, and microstructure. Under the preheating condition of 250 °C, the coating achieved a crack-free state with the highest amorphous phase content, the finest grain refinement, and precipitation of hard phases such as WC, Cr7C3, and M23 (C, B)6. This is attributed to the fact that the preheating effectively reduced the temperature gradient and thermal stress and optimized the solidification behavior of the melt pool. The coating exhibits an optimal overall performance with a microhardness of 813.7 HV0.3, minimal wear volume, and the lowest corrosion current density. Too high a preheating temperature leads to grain coarsening, dissolution of hard phases such as WC, and property degradation. The proportion of the amorphous phase and the number of cracks together determine the performance of the coating.