Microwave plasma has shown significant advantages in the fields of materials synthesis and chemical catalysis due to its high electron density and power utilization efficiency. To solve the problem that the small reaction area of traditional reactor limits its large-scale application, a three-prism microwave plasma reactor structure is innovatively proposed based on the principle of compressed waveguide. The design adopts a three-port symmetric configuration and introduces a compressed waveguide structure inside the cavity to achieve effective superposition and enhancement of the electric field. In addition, the influence of the port positions and microwave phases on the reflection coefficient and electric field distribution characteristics inside the cavity is systematically analyzed by using the multi-physical field coupling calculation method. The results show that optimizing the port position can reduce the reflection coefficient and improve the efficiency of energy utilization, regulating the port phase can effectively enhance the superposition effect of the electric field, so that the electric field is concentrated and widely distributed in the area of the quartz tube, and the peak field strength is as high as 1.64×10⁵ V/m, which can satisfy the excitation conditions of the large-area plasma, providing a reference for the subsequent research on microwave plasma chemical reaction.