It is a better method to use microwave resonator to measure gas parameters accurately, based on the characteristics that resonant frequency changes along with the different mediums, which pass through the microwave resonator. The energy coupling degree between microwave cavity resonator and external waveguide affects the performance and precision directly. According to the propagation characteristic of electromagnetic wave in waveguide, a novel coupling model has been established. By analyzing electromagnetic energy of resonance coupling hole and waveguide coupling face, we choose optimum coupling match method, to make the resonance export the maximal electromagnetic energy, and avoid owing to electromagnetic smaller energy due to the increasing demands for circuit system. Simulation experimental results show that the best coupling of electromagnetic energy can be get only at the position where the waveguide center is departure a certain distance from the resonance coupling hole center, not the superposition between the resonance coupling hole center and waveguide center.