The polarization crosstalk of a TE/TM mode splitter based on a Ti:LiNbO 3 waveguide crossing is deduced by using a beam propagation method(BPM). Optimizing the configuration of the device, the theoretial crosstalk is less than -23.7 dB(TM) and -22.8 dB(TE). The crosstalks of developed splitters have been measured, and the experimental results are consistent with theory.