Influence of the inner windows on the shielding effectiveness of a cavity with apertures is investigated by using the adjusted transmission line method (TLM). Electric shielding effectiveness is calculated as a function of the opening width of metal windows. It is shown that the electric near-field shielding effectiveness of a cavity with inner windows and apertures is far inferior to that of far-field. Results also show that the near-filed shielding effectiveness increases with the decrease of the opening width of the inner window, and the capacitive windows may lower the resonance frequency while the inductive windows may enhance. Results of the adjusted TLM are in good agreement with the CST simulation results.