A novel integral-equation equivalent-circuit method is proposed for the efficient electrical simulation of the power distribution network in the electronic package and printed circuit board. The total electromagnetic field inside the power distribution network is decoupled into two modes: the parallel-plate mode sandwiched between the power and ground planes and the transmission line mode propagating along the signal traces. An integral-equation method is then proposed to solve the parallel-plate mode. By discretizing the integral equation, the power-ground planes pair is equivalent to a ground network. Finally, this ground network is recombined with the transmission line mode to simulate the entire power distribution network. Meanwhile, the discontinuity due to the thru-hole via is also considered by an analytic formula. The proposed method decomposes the complex 3-D problem into two simple 2-D problems, and then extracts the equivalent circuits. Therefore, it greatly reduces the computing time and memory requirement. Through several examples, its accuracy and efficiency are validated.