In order to improve color gamut of the LCD, a filter based on TiO2-Ag-TiO2-Ag-glass structure is added in the backlight unit to optimize the backlight spectra. The proposed filter can only transmit the red, green and blue light in visible light range, while reflecting other color light. The effect of a filter on the color gamut of LCD is simulated by TechWiz 1D software. When the proposed filter integrated with pc-WLED, KSF-LCD and QD backlight, the color gamut of LCD can be improved from 72% to 102.6% NTSC, from 92.3% to 113.8% NTSC, and from 104.3% to 1199% NTSC, respectively. When the incident angle of light increases from 0° to 40°, the color gamut with three backlight changes a little. The simulation results show that the filter based on MDM structure has a smaller angle dependence, so it is a very effective method to realize wide color gamut LCDs. However, the light utilization rate is very low, and the light efficiency of the three backlights is 265%, 35.4%, and 40.7%, respectively.