Considering the characteristics of the plume temperature measurement of solid propellant rocket engine, a new emissivity assumption was presented. Based on the assumption, a new data processing method of the multiwavelength pyrometer was introduced. By processing the measured data of the multiwavelength pyrometer at two different time, the true temperatures and spectral emissivities at two different time can be simultaneously calculated. The experimental results show that the temperature differences between the calculated values and the theoretical ones provided by the rocket engine designer are within ±20K. The above mentioned method is a practical data processing method of solid propellant rocket engine plume temperature.