An improvement to the traditional Fabry-Perot tunable liquid crystal filter which makes liquid crystal as the intracavity dielectric layer is presented. Placing one polarizer on each side of the filter, and the angle between optical axis of the two paralleled polarizers and the molecular major axis of liquid crystal is 45°. The incident beam first generates multiple-beam interference, then polarized beam interference. It is concluded that the resonance wavelength is given by 2(n ∥-n ⊥)Lcosθ=mλ. So the disadvaisntage that the two spectra can′t be distinguished of the traditional Fabry-Perot tunable liquid crystal filter is overcomed. Near 1550 nm, the characteristics of this tunable filter is analysed under 1.6 V, 1.9 V, 2.3 V.