In order to solve the underdetermined equation group of multiple spectral channels in a spectral pyrometer, the idea of optimization is introduced, and the problem of solving multispectral true temperature is transformed into a multi-objective minimization optimization (MMO) problem. The true temperature can be solved without assuming the function relationship model between spectral emissivity and other physical quantities. The inversion accuracy of the proposed method is equal to that of the second measurement method, and the inversion speed is greatly improved. With the aid of the actual measurement data of rocket exhaust plume, the MMO method is used to realize the inversion of the true temperature of the rocket exhaust plume.