The reaction chamber is used as the core of the optical fiber preform by microwave plasma chemical vapor deposition. The structure directly affects the electromagnetic field distribution in the reaction chamber, which affects the plasma state. Therefore, it is necessary to study the structure of the reaction chamber. In order to obtain plasma with higher density and better uniformity in the reaction chamber, a two-way microwave coupled reaction chamber structure was proposed. Firstly, the electric field distribution in the reaction chamber under different reaction chamber structural parameters was simulated and calculated. The effect of the reaction chamber structure and working pressure on the plasma distribution in the quartz tube was investigated by plasma emission spectroscopy using oxygen as the working gas. The results showed that the two-way microwave input method produces a strong electric field coupling enhancement effect in the central region of the quartz tube. The inner diameter of the reaction chamber had a great influence on the electric field distribution state. When the inner diameter of the reaction chamber was 86 mm, the electric field distribution in the quartz tube exhibits axis symmetry, and the plasma density in the axial center region was the largest. When the distance between the two rectangular waveguides was 61.2 mm, and uhe length of the reaction chamber was 202 mm, the intensity and uniformity of the plasma in the reaction chamber were optimally distributed. It was also found that when the pressure rises from 1.8 to 2.8 kPa, the spectral intensity of the plasma in the reaction chamber decreased, but the change near the inner wall of the quartz tube was not obvious, which was caused by the energy loss caused by the increase of particle collision probability in the plasma and the high temperature of the tube wall.