Temperature strongly affects the conversion efficiency, local reactivity, and particle stabilization of the reactions, particularly at high temperature. This study integrated infrared thermography with a high-temperature hot stage reactor to develop a methodology of continuous temperature measurement for a single particle reaction at high temperatures. The external optical transmission transmittance and temperature field core were corrected, and the relationship between the measured temperature (Tr) and true temperature (T0) was obtained by adjusting the sample emissivity. Sample temperature with known emissivity at elevated temperature was measured with 1% deviation. The instantaneous temperature measurement for the gasification process was carried out and the overall emissivities of coal particles were corrected. Temperature distribution and variation on the coal particle surface during the gasification process were further obtained and consistent with the previous research results. The proposed online temperature measurement method provides a new approach for gaining insights into particle reaction states. •Established online high-temperature measurement system for millimeter particle.•Algorithmic relationship between measured and true temperature was obtained.•In situ measurement of temperature distribution was recorded.•<8 K repeatability deviation for reaction temperature measurement.