Abstract: A photonic crystal resonator cavity operating at TM₃₁₀ mode for multi-beam klystron (MBK) is studied. It is found that setting a monolayer photonic crystal structure in the centre of cavity defect can suppress the parasitic modes near the operating mode. Further, in order to fabricate the resonator cavity easily, the resonator cavity only setting a monolayer photonic crystal structure in the centre is adopted which can not only suppress the parasitic modes, but also enlarge the frequency interval between these modes and the operating mode. By means of HFSS, this kind of resonator cavity in X-band is simulated, which shows the availability of the parasitic-mode-suppressing method.