By introducing big screen positioning methods based on light reflection, the principle of visual touch is explained and the big touch-screen system is built. For large-scale trend of FPD (Flat Panel Display), touch detection method based on small-signal is studied. Row selected, background followed and screen heavy pressure removal method are proposed. It is demonstrated in the experiments that large screen touch system based on light reflection achieves full-screen drawing on a 100-inch screen without wearing any marks or sensors and there are not drawing breaking and skipping phenomena. The design of touch system also lays the foundation for large-size volumetric human-computer interaction.