In this paper, the photonic crystal loaded rectangular resonant cavity is studied. Because the conventional way cannat ensure single-mode operation in the cavity when operated with higher-order modes, characteristic impedance method and perturbation method are employed to change the structure of the conventional photonic crystals cavity. The improved cavity that can work with a single higher-order mode is realized. On this basis, the conventional photonic crystal shielded cavity, which can work with a mode of TM530, is reformed by the new method. The new structures are analyzed by using analytical methods and HFSS. The physical reasons of these methods solving the problem of mode competition are also analyzed. The investigation reveals that the method can effectively suppress mode competition and realize single-modc operation in the cavity when operated with higher-order modes.