Abstract: It is very difficult to realize the shielding effectiveness (SE) simulation of complex enclosures consisting of quantity vents, slots and inner components. Numerical simulation always faces the challenges from fine structure modeling, uncertainty of material parameters, huge number of grids and slow convergence of calculation. Therefore, many literatures just refer to the SE simulation of empty enclosures with simple holes or slots. However, the resonance characteristics between an empty enclosure and an enclosure crammed with components are completely different, which inevitably lead to the mismatch of simulation and experimental results. This paper introduces the background transmission loss to simulate the electromagnetic energy absorption in enclosures by internal components. A link between background transmission loss and component absorption loss is established based on the Quality Factor (Q factor) testing theory in reverberation chambers, and finally it determines the parameter setting of background in return. Proposed modeling methods accurately describe the resonance characteristics in real enclosures and reduce the dependence of numerical simulation on material parameters and mesh grids, greatly enhance the simulation efficiency, and consequently result in good matches of simulation and experimental results.