Using the three primary colors RGB in liquid crystal display (LCD) is not sufficient to display the details required in high-end applications. By using more primary colors, such as adding yellow, the four primary colors RYGB is formed, the color gamut could be improved remarkably. Firstly, the electro-optical properties of LCD are researched. And then, the influence of spectral bandwidth on LCD electro-optical properties is studied deeply. Driven by 1 kHz fixed frequency, the transmittance of LCD changes with the changing of the voltage. With the changing of the driving voltage, the transmittance of LCD changes dramatically. Furthermore, by selecting the wavelengths corresponding to the four primary colors, the electro-optical property curves are researched, and the curves changing accords with the voltage changing trends. Through researching the transmittances of four primary colors wavelength under different bandwidth, it is found that the transmittances of three primary colors such as red, yellow and green is little influenced by the bandwidth, but the transmittance of blue primary color is greatly influenced.