A new method for measuring the emission spectra of the YAG phosphor under blue light excitation was proposed. Utilizing the Gaussian function and Fermi function fitting and simulated annealing optimization techniques, mathematical fitting functions for matching mixed emission spectrum were obtained. And so, the emission spectrum of the YAG phosphor was separated, further its luminescence properties can be characterized. Our experiment results proved that its characteristic parameters present non-negligible differences. The optical metric differences such as those in quantum efficiency, light emitting efficiency and energy efficiency are higher than 1%. The difference in CCT (correlated color temperature) is up to tens of K, and chromaticity indicators also produce differences in chromaticity coordinates, dominant wavelength and color purity. But little change is there in the half-peak bandwidth, and peak wavelength is basically unaffected. All these show that the established method can improve the measurement accuracy of YAG yellow phosphor emission spectrum under practical conditions.