Abstract: In this paper, aiming at the limitations of the equivalent admittance model of the dielectric plate in the existing shielding efficiency topology model of the built-in dielectric plate cavity, considering the influence of the height, thickness and position of the dielectric plate on the equivalent admittance model of the dielectric plate, the topological model of Baum-Liu-Tesche (BLT) equation of the shielding efficiency of the built-in dielectric plate cavity is established. The transmission line method and the topological model of BLT equation are compared with the simulation results to verify the accuracy and validity of the topological model of the BLT equation. Taking the open-hole cavity with a length of 300 mm, a width of 300 mm, and a height of 120 mm as the research object, the effects of dielectric plate height, thickness and position on cavity shielding efficiency under three different resonant modes are explored. The results show that changing the height of the dielectric plate keeps the resonance mode of the cavity unchanged, which only affects the shielding efficiency of the cavity. The installation of the dielectric plate inside the cavity can enhance the cavity shielding ability, but the thickness of the dielectric plate is too large, which will weaken the cavity shielding ability within a certain range. Installing the dielectric plate 0 mm~58 mm away from the cavity hole array will weaken the cavity shielding ability, and installing the dielectric plate 100 mm~150 mm away from the cavity hole array will enhance the cavity shielding ability. This conclusion can provide an effective reference for the design of electromagnetic shielding of shielded chassis.