Abstract: To address heat accumulation during laser deposition of thin-walled structures, which may cause microstructural coarsening and property degradation, a heat and mass transfer model for a multilayer laser-cladded thin wall under substrate water-cooling conditions was developed and experimentally validated. The evolution of solidification conditions at the trailing edge of the molten pool, including temperature gradient, solidification rate, and cooling rate, was systematically analyzed. The results show that substrate water cooling significantly regulates solidification behavior at the molten pool trailing edge. With increasing convective heat transfer coefficient of the substrate, the temperature gradient and cooling rate increase markedly, while the solidification rate changes only slightly. As the deposited height increases, the direct heat dissipation effect of the substrate weakens and heat accumulation becomes more pronounced, leading to an overall decrease in temperature gradient and cooling rate, whereas the solidification rate remains relatively stable. Substrate water cooling effectively increases the average cooling rate, mainly during the early deposition stages. The maximum cooling-rate difference decreases from 2.5×10³K/s at the bottom layer to 1.0×10³K/s at the top layer.