Microwave plasma chemical vapor deposition (MPCVD) is an important method for synthesis of high-quality diamond film due to its high plasma density, good controllability and high cleanliness in the deposition process. Based on the theory of resonant cavity and three-dimensional full-wave electromagnetic field simulation, the microwave plasma cavity, mode converter and substrate holder shapes, which have a great influence on microwave coupling efficiency and electric field distribution, were designed and optimized. By testing and monitoring the key parameters in the microwave transmission system, influence of the tuning variables on diamond deposition were analyzed. With the MPCVD device proposed in this paper, high quality diamond films were deposited with an effective circular growth area with a diameter of 50 mm at a growth rate of 10 μm/h to 25 μm/h. The characterization results show that the single crystal diamond films have optical transmittance close to theoretical threshold and an excellent crystalline structure, with low impurity contents of nitrogen and silicon.