Abstract: A problem about the optimal extinction diameters of fine water sprays for thermal radiation shielding is researched. Considering the optics and dynamics properties of the sprays droplets, the index of thermal shielding is defined as a criterion to measure the extinction capability of dynamical droplets. The polydisperse sprays are equal to monodispersion with Sauter diameter in the model construction. The Planck meaning method is used to obtain the radiative property parameters of sprays taken as grey body. The massive sums of repeat calculating of Mie efficiency factors are avoided by adopting the method of data invoking which are calculated and saved in an array in advance and linear interpolation. According to the evaluation method defined in this paper, the optimal extinction diameters obtained based on thermal shielding index are far greater than that obtained based on the optics properties.