According to the electro-optic effect of liquid crystal and the theory of Lyot filter, the tunable liquid crystal filter is designed. Its feasibility is tested theoretically and experimentally. In order to overcome its disadvantages in infrared band, Fabry-Pérot tunable liquid crystal filter is designed and its analogue is given. It is proved that the Fabry-Pérot tunable liquid crystal filter has great tuning range and narrow band.