The influence of the reflectance phase shift of the reflector on the peak wavelength in a liquid crystal Febry-Perot etalon (LCFPE) filter is discussed. In the tunable LCFPE filter, the peak wavelength is selected by adjusting the applied external voltage. The peak wavelength shifts due to the wavelength-dependent phase shift of the reflector. A set of simple formulas which can give quickly the phase shift are derived. The simulation results indicate that the reflective phase shift can not be ignored in the design of a LCFPE filter as it plays an important role in determining the peak wavelength and the applied voltage of the device. Using these formulas, a tunable LCFPE filter in the optical communication C-Band is designed.