Lithium niobate, dueto its strong electro-optic effect, is an excellent material for high-performance optical modulators. Hybrid integration of thin film lithium niobate and silicon photonic circuits makes it possible to fully exploit potentials of the two material systems. In this paper, we introduce a detailed design procedure for silicon and lithium niobate hybrid integrated modulator using coplanar line electrodes based on Mach-Zehnder interferometer push-pull configuration. A multiphysics model for the crossing section of the modulation section is proposed and analyzed. The results show that optimizing solely the <inline-formula><tex-math notation="LaTeX">$V_{\pi } L$</tex-math></inline-formula> product would not lead to the best 3-dB bandwidth for a certain half-wave voltage due to the increased microwave losses. There exists an optimal ground-signal electrode gap value, which is about 8–9<inline-formula><tex-math notation="LaTeX">${\,\mu m}$</tex-math></inline-formula> for the present modulator structure. For these optimized structures, 3-dB bandwidths can reach 45 GHz and 137 GHz with half-wave voltages of 2 V and 4 V, respectively, for a lithium niobate waveguide total thickness of 600 nm and a ridge height of 200 nm.