In this paper, Ni-based amorphous composite coatings were fabricated under different heat inputs on a mild-steel substrate using laser cladding with coaxial powder feeding. The microstructure of the coating was studied using a scanning electron microscope (SEM), X-ray diffraction (XRD) and transmission electron microscope (TEM). The effects of the heat inputs on the amorphous-phase forming ability of the Ni-based alloy was investigated systematically with experimental and numerical simulation methods. The results show that there was no amorphous phase in the coating when the heat input was 131.3 J/mm. The amorphous-phase fraction increased with a decrease in the laser-cladding heat inputs from 81.3 J/mm to 50.0 J/mm. Then a 3D thermal finite-element (FE) model was built to simulate the temperature field of coaxial laser cladding at different heat inputs using the element birth and death technique. Detailed 3D transient thermal analyses were performed on temperature-dependent material properties. The proposed model was validated with the experimental results. It was found that a decrease in the heat input leads to a lower high-temperature residence time and a higher cooling rate of the melted pool. Consequently, a low heat input can be considered as a necessary condition for the formation of the amorphous phase during the laser-cladding process.