The underwater laser cladding layers of Ti-6Al-4V titanium alloy were prepared using underwater wire-feeding laser cladding (UWLC) technology under different heat inputs and gas flow rates. At optimum process parameters, the uniform underwater cladding layer without oxidation was obtained, and the cracks inside layer were restrained successfully. The deposition morphology, grain characteristics, microstructure and properties of the cladding layer were analyzed, and the mechanism of crack elimination was investigated. With the increase of heat input, the grain size and thickness of the α lath increased, while the amount of α' decreased, and the oxidation phenomenon had also improved. The sizes of columnar β-crystal grains and spherical β-crystal grains both increased. The microstructure of the different heat input layers was composed of lamellar α phase and acicular α′ phase with different sizes. Besides, the reduction of the cooling rate inhibited the unidirectional growth of acicular martensite, so that the martensite arrangement was close to a disordered state, and the deformation of the cladding layer was more uniform, which effectively prevented the cracks and melt expansion. The corrosion resistance of the coating was improved, and the microhardness was reduced.