Resonant properties of HE111 mode of a complicated microwave cavity with ceramic material, used in rubidium clock, are studied by mode matching method. The microwave cavity works at a certain frequency such as 6835 GHz by accommodating a glass bubble containing rubidium vapor. To make electromagnetism focus on the centre of the cavity for energy exchange and further miniaturize the cavity to a large extent, a ceramic dielectric ring is installed in the inner layer of the glass bubble. In order to study main factors influencing resonant characteristics, resonant frequencies of HE111 mode are calculated through eigen equation and compared with simulated results. The results show that theoretical computations are in good agreement with finite element simulations. In addition, the effects of loaded dielectric and rubidium vapor on resonant frequency are also analyzed. This work is of great significance for the miniaturizing of the cavity and theory perfection in atomic clock.