This paper proposes a novel in-situ measurement method for surface temperature and normal spectral emissivity of thermally oxidized TC-4 high-temperature alloy, and establish a model for solving temperature and normal spectral emissivity based on the thin film interference mechanism. The results indicate that the temperature calculations are in good agreement with the measurements from surface thermocouples, with a maximum error of less than 3 K within the temperature range of 473 K to 973 K. The maximum error in the total directional emissivity within the wavelength range of 2 μm to 14 μm is 0.014. The measurement method and apparatus exhibit excellent repeatability, with the standard deviation of multiple measurements of normal spectral emissivity at temperatures of 473 K and 973 K is 0.035, 0.019, respectively. This method provides an effective solution for addressing the issue of fluctuating emissivity on the thermally oxidized surface of TC-4 high-temperature alloys over service time.