In view of the large half-wave voltage of the lithium niobate thin film Mach-Zehnder electro-optical modulator, the finite element method was used to analyze the coupled mode theory of the directional coupler in detail, and the key structural parameters of the electro-optical modulator were optimized. The results show that the optical power difference between the two interference arms and the coupling length of the waveguide present a sine function distribution, and the minimum coupling length of the waveguide increases as the coupling distance increases. When the modulation arm length is 2 cm and the light wavelength is 1550 nm, the half-wave voltage V_π calculated by the simulation is 0.9 V. The half-wave voltage length product is 1.8 V·cm, and the extinction ratio reaches 26 dB. Through the cross-section analysis of the modulation arm, the electrostatic field component E_x,the electric displacement vector D_x,and the optical mode distribution are obtained, and the electro-optical overlap integral factor Γ is calculated to be 0.586. Based on the above simulation results, compared with the half-wave voltage(1.4 ~ 10.2 V) of the existing electro-optical modulator, the optimized electro-optical modulator has a lower half-wave voltage. This in turn makes the device power consumption lower, conducive to large-scale optoelectronic integration.