In order to realize the electro-optic tunable filter with high rectangular degree, low sidelobe, electro-optic tuning reverse derivation and passband width tuning, the N-stage interdigital electrodes distributed periodically are provided with independent voltages V on the x-cut, y-propagation Ti-diffused LiNbO3. The induced periodic electrical field could achieve conversion of the quasi-TE mode and the quasi-TM mode and also the wavelength filtering. The finite impulse response (FIR) digital filter network is composed of this optical path. And there is a one to one correspondence between the transmission function H(z) and the voltage matrix V. Using z-transform and method of undetermined coefficients, the voltage matrix V is derived to realize the electro-optic tuning reverse derivation. The tunable passband width in the free spectrum range (FSR) can be realized with different voltage matrices V. This method is proved to be feasible through the simulation. When N is equal to 17, side mode suppression ratio (SMSR) could reach 25 dB. Also, the rectangular degree of the filtering increases with the increase of cascading stage N.