The surface temperature of continuous casting slab in water spraying zone is a key parameter of production control. However, the accuracy of radiation temperature measurement is greatly affected by water vapor at high temperature. In this paper, a mathematical radiative heat transfer model combined with an analytical radiation thermometry model was developed. The hot water vapor absorption coefficient at a spectral resolution of 25 cm−1 as an input parameter to the radiative heat transfer model is obtained by employing the statistical narrow-band model along with gray-band approximation on the basis of the updated database HITEMP 2010. The proposed model was applied to quantitatively calculate the surface temperature, obtained by ratio and single waveband thermometer for infrared waveband from 50 cm−1 to 11250 cm−1, of assumed gray strand covered by hot water vapor. Detailed simulation studies were performed considering the following parameters: concentration of the hot water vapor, reflectivity of the target surface, background temperature of the surrounding surfaces and distance between target and thermometer. A sensitivity analysis was put forward trying to ascertain how the proposed model depends on those parameters. As the results indicate, both hot water vapor and reflected ambient radiation greatly affect the measurement accuracy of the radiation thermometry. The temperature errors caused by hot water vapor are spectrally selective. The most suitable spectral regions for ratio and single waveband thermometer operating and a criterion for choosing the optimal wavelength pairs for ratio thermometry were put forward. The analyses in the paper may provide necessary theoretical supports for the design and application of a radiation thermometer in the presence of hot water vapor.