In order to improve the ability of the electronic components in the equipment to resist electromagnetic interfer-ence from the environment and other internal components,the transmission line method(TLM)is applied in this paper to con-vert the model of double layer metallic enclosure with apertures and load circuit boards to circuit diagram,and the calculation formula of enclosure center shielding effectiveness is deduced with the circuit diagram. The Matlab is adopted to generate the shielding effectiveness curve. The model is established and simulated with the simulation software CST. The simulation results match well with the Matlab output curve,which verifies the correctness of the calculation formula. The effects of some factors (such as the PCB size,quantity,putting way and the distance to aperture) on shielding effectiveness are analyzed by CST. In order to fit the realistic situation,the circuit board loading the integrated operational amplifier circuit was used to analyze the shielding effectiveness and the effect of shielding enclosure on the function of the circuit board. Finally,some approaches to im-prove shielding effectiveness are proposed.%为了提高设备中电子元件抵御来自外界和内部其他元件的电磁干扰，根据传输线理论，将双层加载电路板屏蔽腔体模型转换为电路图，利用电路图推导出腔体中心屏蔽效能的等效公式。利用Matlab生成传输线法屏蔽效能曲线，并通过仿真软件CST建模仿真，仿真结果与Matlab输出曲线良好吻合，验证了公式的正确性。运用CST研究了一些因素如电路板大小、数量、放置方式以及距孔缝的距离对屏蔽效能的影响。为了更加贴合实际，采用加载集成运算放大电路的印制电路板来研究腔体屏蔽效能以及腔体对电路板功能的影响，最后提出了一些提高屏蔽效能的方法。
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