In recent times water spray has become a key agent in fire protection because it is low cost and environmentally friendly. In this paper, the spectral absorptance and transmittance of water droplets and the evaporation flux have been calculated in a homogeneous water spray using Mie and radiation transfer theories. The relationship of the evaporation flux, the wavelength, initial temperature, mean diameter, and the length of the water spray were calculated. The results indicate that the droplets with smaller particle size (3 < D < 7 mu m) are suitable for a water curtain, because they have largely evaporation flux. However, that does not mean the smaller the better. Therefore, we should choose a more suitable size to form a water spray curtain to protect the objects. The above analysis may be useful for the simulation of the radiation attenuation in water sprays using CFD.