The quality factor and the resonant frequency of a resonant cavity are the key factors that need to be considered in the process of resonator design. The wall of cavity is composed of conductor materials which are effective tools to generate high-frequency oscillation. The microwave cavity is widely used. From the perspective of the circuit, it has almost all the properties of LC resonance unit, such as mode selection. Therefore, it is widely used in filters, matching circuits, and antenna design. In industrial applications, the demand for high-frequency resonant cavity is relatively large. A traditional method can increase the resonant frequency of the resonant cavity by reducing the size of the cavity or using the high-order modes. However, as both approaches have their limitations, the design results are not ideal. By combining theoretical calculation and simulation, the factors that affect the resonant frequency of the resonator are analyzed. The results show the relationship between material properties of the filling medium and the resonant frequency of the cavity. Theoretical calculations show that when the left-handed materials are used as filling materials in the cavity, the resonant frequency can be increased without changing the size of the cavity. The results of high frequency structure simulator also prove the above result. Therefore, the resonant frequency of the resonator cannot be limited by the cavity size. It can be seen from the data that compared with reducing the size of the resonator or using high-order modes, filling left-handed materials can improve resonant frequency to a greater extent. The obtained conclusion shows a further progress compared with the traditional theory and provides a theoretical basis for the exploration and design of novel resonators.