Emissivity, as a key parameter to characterize the radiation properties of an object, and its accurate measurement is of great value for high-temperature target identification, characterization of material modification, and regulation of metal smelting process. The emissivity measurement by multispectral radiation method has become a research hotspot because of its advantages of non-contact and fast measurement speed, and its measurement accuracy is determined by the solution accuracy of the underdetermined system of equations. At present, the research on the solution accuracy of the underdetermined system of equations mainly focuses on the error of the equation solving algorithm, ignoring the measurement error of the spectrometer itself, which leads to the failure of controlling the system error in a reasonable way. In this paper, based on the assumption of retardation with wide application range and high measurement accuracy, the influence of the number of spectral channels and signal-to-noise ratio on the emissivity measurement error under different conditions is simulated, the parameter configurations of the spectrometer under the corresponding conditions are determined, and the effect of emissivity measurement is experimentally verified. The experimental results show that, using the multispectral radiation method based on the slow-change assumption, the number of spectral channels of the spectrometer should be not less than 400 and the signal-to-noise ratio should not be less than 1000 in order to make the blackbody emissivity measurement error less than 1%; for the targets with complex emissivity changes, the spectrometer should have at least 1,000 spectral channels and signal-to-noise ratios of more than 1,200 in order to make the measurement error less than 1%. Comprehensive consideration of the algorithm error and spectrometer parameter matching relationship is the key to rationally control the system error, and more accurate emissivity measurement results can be obtained, which provides a new basis and solution for the application of multispectral radiation method to accurately measure the emissivity, which is of great significance for the accurate identification of high-temperature targets and the application of the related fields.