This study explores the mass transfer behavior in the coaxial powder-fed laser cladding of AlCoCrFeNi high-entropy alloy (HEA) coatings on 45# steel under varying laser power conditions. The dilution rate was quantified, and the elemental distribution was analyzed. A coupled thermo-fluid-mass transfer numerical model was developed to investigate the flow dynamics within the molten pool. The results indicate that an increase in laser power leads to higher substrate dilution, with dilution rates of 17.9%, 19.8%, and 22.7% corresponding to laser powers of 800 W, 1000 W, and 1200 W, respectively. Microstructural analysis reveals a transition from planar to columnar and equiaxed grains along the cladding depth, governed by thermal gradients and solidification rates. Numerical simulations further demonstrate that intense convective flow, driven by thermal buoyancy and Marangoni effects, promotes elemental homogenization within the melt pool, mitigating compositional gradients. These findings enhance the understanding of substrate dilution mechanisms.