This paper presents an enhancement in the shielding effectiveness (SE) of an open cabinet containing digital modules using a ferrite sheet when an external electromagnetic source impinges on the gate of the cabinet. We perform an electromagnetic analysis on the inner space of an open cabinet using a mode-matching method, which proceeds in the following order: the separation of the analyzed region, the representation of fields, and the enforcement of the boundary conditions on the tangential field continuities between the separated regions. To be specific, it is efficiently utilized in the representation of electric and magnetic fields by Helmholtz's equation in conjunction with both the separation of the variable and the Fourier transform. After confirming the convergence of the solutions to the set of simultaneous equations, we investigate the SE in the open cabinet in terms of a frequency, an incident angle, and the number and the size of slits. The computed results provide us with useful information for avoiding electromagnetic interference. We then examine the improvement of shielding performance by inserting a ferrite sheet at the gate of a cabinet. We verify that the usage of the ferrite sheet with high permeability can be a preferable solution for removing the undesirable problems caused by an external electromagnetic source.