Steel is exposed in air at elevated temperatures in production processes. Surface oxidization is propagating through the whole period of manufacture. To measure the temperature of a steel surface in production as accurately as possible, the relationship of the spectral emissivity versus temperature must be determined in the oxidizing environment. For this reason, in this paper, a study of the effect of surface oxidization on the spectral emissivity over the temperature range from 800 K to 1120 K by taking DC01 steel as an example is described. In this experiment, the surface temperature is measured by averaging two R-type platinum–rhodium thermocouples. The radiant energy coming from the steel surface is received by an InGaAs photodiode detector. The experimental setup works at a wavelength of 1.5 \(\upmu \mathrm{m}\) with a bandwidth of 20 nm. Two kinds of relationships between the spectral emissivity and the temperature are studied in detail in the oxidizing environment. One is that the spectral emissivity varies with the heating-duration time at a given temperature. The other is that the spectral emissivity varies with the temperature at a given heating-duration time. Resonant peaks of the spectral emissivity are observed during the whole heating period. The behavior of the spectral emissivity is discussed when the oxidization film on the specimen surface is grown, in particular at the early stages of the heating duration. Analytical formulas of the spectral emissivity versus temperature are derived for different heating-duration times: (30, 60, 90, 120, 180, and 240) min. The conclusion is gained that the coefficients of the analytical expressions between the spectral emissivity and the temperature are different for the measurements obtained at the different heating-duration times, although the same logarithm-functional form is suitable for fitting all the experimental results obtained in the present work.