This article mainly deduces the new method of the shielding effectiveness, using transmission lines method to compute power, then we can use power to compute the shielding effectiveness with PCB in it. Because power takes into account the attenuation of electromagnetic fields in the high-frequency, compared to voltage method, we can get that power method calculating shielding effectiveness is accurate. This article is useful to the simulation of the microwace oven and other project, such as computer chassis.