Plasma-filled slow-wave devices provide a new way to develop high efficiency and high power vacuum-electron microwave sources, but their theoretical analysis and simulation is difficult. This paper introduces the wheel spoke antenna to excite signals for analyzing the dispersion characteristics of resonant cavity with plasma-filled metallic photonic crystal slow-wave structure (SWS). Influences of parameters of the SWS and plasma density on dispersion characteristics of the SWS are studied. Results show that there is little difference in dispersion characteristics obtained by wheel spoke antenna excitation of signals and other methods without plasma filling. When plasma fills in the SWS, the frequency of zero mode is consistent with the previous results obtained by other methods. Hence, both the results with and without plasma filling demonstrate that the wheel spoke antenna signal-excitation method is effective. Moreover, decreasing the thickness of wheel spoke antenna properly and the distance between the antenna and reflection surface of the metal plate can reduce the wheel spoke antenna influence on the cavity resonance frequency. Furthermore, thicker antenna can excite the slow wave field easily, while thinner antenna can excite the resonant mode easily. Besides, the outer radius and thickness of the SWS plate have little influence on the dispersion characteristics, while the period length and the inner radius of the SWS plate have greater influence on the dispersion characteristics. In addition, the dispersion curves of frequency and phase velocity will move to higher frequency regions with the increase of plasma density. Further, the influence of plasma filling on low-order modes is greater than that on higher order modes. It is also found that the higher-order mode operation can reduce the size of cavity and the velocity of the electron beam.