In this study, the IES TM-30-18 gamut index is used to characterize the gamut area of LED sources. Simultaneously, the spectra of 20 sample light sources at four color temperatures are simulated by the superposition of monochromatic light spectra in a 380--780 nm band with a 5-nm interval, and the change of the gamut area is calculated. The results show that the color gamut area of a light source increases after the monochromatic light of 380--465 or 605--780 nm superimposes on the spectrum of each sample light source, and decreases with the monochromatic spectral of a 470--600 nm band. A light box matching is performed to match the superimposed spectrum, and the visual experiment of color brightness and preference is conducted. The experimental results show that the brightness and color preference of the spectra with an increased color gamut area are better than those with a decreased gamut area. When two monochromatic lights with different peak wavelengths and amplitude powers are superimposed at the same time, the color temperature of the light sources can be kept substantially unchanged, whereas, the color gamut can be increased by 2--10. The relationship between the spectral distribution and gamut area is obtained by simulating the source spectrum after superimposing monochromatic light, which is of great significance for optimizing the color gamut area and visual effects of the LED light sources.