Abstract: A novel method using multi-port network theory, and considering the transmission line in cavity with apertures as an multi-antenna system is proposed based on reciprocity theory to fast predict the maximum induced current at the ports of the transmission line in cavity. Since this method does not use the direct full-wave simulation algorithm to calculate the induced current, the calculation efficiency is obviously improved. The maximum induced current of different situation is discussed. The result is compared to the direct full-wave simulation method, and proves that this method is feasible.