The theoretical basis of the national primary standard apparatus of spectral irradiance in China is Planck’s law revealing a real quantitative relationship between wavelength, temperature and spectral irradiance. The spectral irradiance comparison method is used to preserve and transfer the spectral irradiance standard by halogen tungsten lamp. Moreover the temperature measurement of a blackbody is the main source of uncertainty in the realization of spectral irradiance. For a long time in the past, a variable high-temperature blackbody was used as the primary radiation source for realization of spectral irradiance scale, and the temperature measurement of the blackbody was realized by a pyrometer traceable to the fixed-point temperature scale blackbodies of NIM. In order to meet the needs of high accuracy measurement of spectral irradiance in the fields of earth observation, meteorological remote sensing, climate change monitoring and ocean color detection in China, National Institute of Metrology (NIM) established a 14 mm diameter WC-C high-temperature fixed-point blackbody (HTFP) system, which was used as the primary radiation source to realize spectral irradiance scale directly. This method can shorten the traceability chain and reduce the temperature measurement error. In the experiment, the data obtained by a pyrometer are only the relative distribution of the blackbody temperature rather than the absolute value. In order to obtain the absolute temperature of a WC-C HTFP blackbody which can be used for the realization of spectral irradiance, it is necessary to use the point of inflection (POI) temperature of the melting temperature plateau curve for comparison calibration. So it is important to calculate and evaluate the POI temperature reasonably. Unlike a small-area WC-C HTFP, the melting temperature plateau curve of a large-area WC-C HTFP has a longer duration and has greater temperature variation, so traditional POI calculation methods, which are widely used in small-area HTFPs, are no longer applicable. So this paper proposed a selective multiple fit methods calculating the POI of a large-area WC-C HTFP with a 14 mm inner diameter. The influences of selective criterion, data smoothing and fitting range on the calculation results of POI were investigated. The maximum discrepancy between the new and traditional methods was 0.001 and 0.633 K, introducing 0.000 3% and 0.20% spectral irradiance measurement errors at 500 nm respectively. Using small-area WC-C and Re-C fixed-points with 3 mm inner diameters to investigate the validity of the new method. The results showed that the maximum discrepancy between the new method and the average value of the three traditional methods was -0.007 and -0.001 K, introducing 0.002 2% and 0.000 3% spectral irradiance measurement errors at 500 nm respectively. Compared with the three traditional methods, the new method can effectively reduce the temperature error and improve the realization accuracy of spectral irradiance. It is more suitable to calculate the POI temperature of a large-area WC-C HTFP blackbody.