Based on a simple theoretical model for Ti in diffused LiNbO3 optical waveguide with graded-index distribution, the optimal configuration parameters for TE/TM mode splitter of directional-coupled structure with metal clad have been obtained by using the beam propagation method. The theoretical crosstalk is less than -20.8 dB (TE mode) and -21.3 dB (TM mode) for λ=1.523 μm.