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Mist cooling concept has been considered for cooling turbine airfoils for many years. This concept has been proven experimentally as an effective method to significantly enhance the cooling effectiveness with several fundamental studies in the laboratory under low pressure and temperature conditions. However, it is not certain the same performance can be harnessed in the real gas turbine environment under the condition of elevated temperature, pressure, heat flux, and Reynolds number. This paper aims at validating a CFD model against experimental results in a circular tube and then applies the validated CFD model to simulate mist/steam cooling performance at elevated gas turbine working conditions. The results show that the standard k-ε and a RSM turbulence models are the best-suited model for this application. The mist with smaller droplet diameter is found achieving higher cooling enhancement than the flows with bigger droplets, while mist with a distributed droplet size matches the data slightest better than with uniform droplets. Both the wall-film and the reflect droplet boundary conditions are employed and their effects on the cooling result is not significant at the studied cases. The validated CFD model can predict the wall temperature within 2% in steam-only flow and 5% in the mist/steam flow. Applying the calibrated CFD model to the actual gas turbine working environment shows that the mist/steam cooling technique could harness an average 50–100% cooling enhancement.