A Mach-Zehnder (M-Z) electro-optical modulator with low driving voltage and high modulation parameters is designed on the X-cut Lithium Niobate (LN) substrate by using a directional coupler instead of traditional Y-shaped branch structure. The working principle and major factors of directional coupler are analyzed and calculated precisely by the finite element method. The major structural parameters of the electro-optic modulator are optimized. The results show that when the light wave propagates along the directional coupler, the energy is alternated back and forth in the two waveguides with a distribution of sine and cosine functions. At the same position, the coupling length increases rapidly with the increase of the gap. When the electro-optical modulation arm length is 3 cm and the optical wavelength is 1550 nm, the half-wave voltage Vπ obtained by the single-arm modulation is 2.22 V and the modulation parameter S21 is -51.13 dB. With the section analysis of modulation arms, the electric field Ex component, the electric displacement vector Dx component and the optical mode distribution are given, and the integral overlap factor is 0.486. It also shows that the ridge waveguide structure can effectively improve the electro-optic efficiency. When the ridge height is 1 μm, the overlap integral factor is 0.714, and the electro-optical overlap integral factor of it is 46.91% higher than the slab waveguide.