Tunable Fabry-Perot filter (FPF) has high practical value in optic communications. Piezoelectric copolymer P(VDF-TrFE) can be used as spacer material in FPF to obtain large tunability in wide range because of its great electrostrictive effect. The filter has been fabricated with the structure of DBR/electrode/spacer/electrode/DBR. The copolymer is partly etched at where the light traverses. The transmittance spectra have been studied. The resonant wavelengths have been modulated by applied voltage. The optical length is measured to be 804 μm, which shrinks under the electric field. The filter exhibits a total optical path-length change of -0757% under an electric field of 140 V. The results indicate that the electrostrictive effect plays a major role.