Abstract: In the colorimetric temperature measurement method, the traditional assignment of complex refractive index assumes a constant refractive index for soot at a designated wavelength, employing either empirical formulas or empirical values for assignment. However, with the study of the mechanism of soot deepens to a smaller scale, this assignment method will lead to a large error in the calculation results of colorimetric thermometry. The relationship between the complex refractive index and soot particle size was established by bridging the physical correlation between the optical band gap and particle size with the relationship between the optical band gap and complex refractive index derived through linear interpolation. The influence of the scale effect of soot particles on the complex refractive index in the colorimetric thermometry was explored through this relationship, and the complex refractive index assignment method was optimized by empirical formulas and experimental values, respectively. The optimized calculated temperature of soot particles was compared with the temperature data measured by laser-induced fluorescence to evaluate the optimization effect of the two methods. The experimental results indicate that the temperature measurement results based on the traditional assignment method of complex refractive index exhibit a relative error of 20% to 50% compared to the experimental results, with the deviation falling within the range of 300 K to 1 000 K. Among the two proposed optimization methods, the approach based on empirical formulas only marginally reduces the relative error by about 5%. In contrast, the optimization method relying on experimental values significantly enhances the accuracy of flame temperature measurements, reducing the relative error by approximately 40%.