The theory of voltage-tunable liquid crystal filter is elaborated,and the change of transmission of BL-009 nematic liquid crystal in visible light band is detailedly analyzed by testing the transmission of liquid crystal using Shimadzu UV-23101 spectrophotometer.Relation curve of transmission with voltage is gained in the wavelength range from 400 nm to 800 nm.The spectral lines at 488 nm wavelength and at 514.5 nm wavelength of argon ion laser are separated.