To improve the filtering performance of conventional Mach-Zehnder Interferometer（MZI）filter

a new structure of filter which combining ring resonator and MZI is proposed in this paper.The transfer function of this structure is first realized by using the Signal Flow Graph（SFG）method.Then the output spectra are simulated and analyzed.The results show that when the perimeter of ring resonator（L

1

、L

2

）and the length difference（Δd）of MZI arms meet the relationship of L

1

=2L

2

=Δd

the improved filter can give rectangular waves approximating flat top and steep edge.Through numerical calculation when choosing the perimeter of ring resonator is L

1

=1 510μm

the free spectral can range up to 2nm

the full bandwidth at halfmaximum is 1.15 nm

the sharpness factor is 1.74 and the quality factor is 1 347.82.Therefore

compared with the conventional filter of MZI’s spectrum

the filtering performance is improved significantly.It is noted that by choosing the polymer materials with excellent photoelectric properties to design the ring resonator and MZI

the spectra are moving to the right side which can be adapted to the different filtering requirements when changing the voltage on the filter.