An equivalent model of magnetic dipole for shielding efficiency (SE) estimation of shielding cavity with slots is presented. SE estimation of cavity is based on the coupling theory of slot and its equivalent model of magnetic dipole. The solution of electric field and the distributing regularity of SE in the cavity can be obtained by the model. The validity is given by the well matched result of the model of magnetic dipole, Micro-Stripes software simulation, and measured data. The method of model analyzes the relationship between SE and the slot number or different observing points in the shielding cavity. The result shows: SE curve decreases totally by the increasing number of similar slot, and it increases by the increasing distance between the axes line of cavity and the observing point, which means that the electric coupling efficiency (CE) is weakened by the increasing distance on the transverse plane of cavity. Furthermore, the model is faster than the method of Micro-Stripes software and FDTD.