The work aims to systematically analyze the influence of the gas flow ratio between CH4 and H2 on the growth of diamond films in a home-made 10 kW MPCVD system. The characteristics of plasma discharging at relative high microwave power were analyzed and studied by plasma emission spectroscopy. The morphology and quality of the deposited diamond films were characterized by scanning electron microscope and Raman spectroscopy respectively in order to determine the regulation principles of the gas flow rate of CH4 in CH4/H2 gas mixture. The increasing microwave power could effectively improve the electron density in the plasma and generate more activated H atoms as well as the species of CH and C2 beneficial to the growth of diamond films. When the microwave power, chamber pressure, substrate temperature and gas flow of H2 were kept at 5000 W, 13 kPa, (950±20) ℃ and 300 mL/min respectively, the diamond films deposited with Q(CH4)/Q(H2)<1.0% exhibited obvious secondary nucleation phenomenon and smaller crystal size. The diamond films grown with 1.0%≤Q(CH4)/Q(H2)≤2.0% possessed well-facets crystals, and the diamond films prepared with Q(CH4)/Q(H2)>2.0% had relative lager crystals containing twins. The increasing microwave power results in an increase in the concentration of activated H atoms as well as the activation level of carbon contained gas. The diamond film prepared at 1.0%≤Q(CH4)/Q(H2)≤2.0% in current experimental conditions has the best quality.