With the rapid development of communication technology, the bottleneck appears in the traditional microelectronics industry. More and more people hope that integrated optical circuit can achieve new breakthroughs. Silicon photonics has attracted much attention because of its advantages in materials and manufacturing technology. Among them, the silicon coupler, as an important silicon passive device, is the key to achieve beam coupling and beam splitting on chip. Multimode interference (MMI) couplers are commonly used as integrated optical devices because of their advantages such as low loss, large process tolerance and large bandwidth. In this paper, based on the self-imaging principle of MMI, guided mode propagation analysis (G-MPA) is used to analyze the mode field distribution in MMI, and a taper multimode interference coupler based on 470 μm high resistance silicon wafer in terahertz band is successfully designed. The coupling efficiency of 93.8% is achieved by FDTD simulation and optimization of its parameters.