In the coming new general optical network, tunable filter is one of important optical devices, and can be used to compose dynamically optical add/drop multiplexer (OADM). Polarization independent integrated acousto-optic tunable filter (IAOTF) is one of devices which have most potential to perform the function[1][2]. Mode splitting and merging are realized by using two symmetrically directional couplers in the IAOTF. Mode polarization is converted by the acousto-optic mode converter when the condition of phase matching is met at the specifically light wavelength. The light wavelength passing the filter can be easily adjusted by changing the frequency of acoustic wave. The Ti diffused technology is adopted to fabricate the device on the X-cut Y propagation LiNbO3 slice. The relation of index difference in the X-cut Y propagation LiNbO3 waveguide with different diffusion conditions is computed in this paper. And the theory of coupling mode is utilized to analyze and design the directional coupler, in which TE/TM mode splitting can be realized. The designed structure is simulated by using BPM software. The extinction ratio of TE/TM mode can be reached 25.1 dB and 24.9 dB respectively.