Power ports of power delivery networks(PDN) in high digital circuit are designed to satisfy the requirements of large transient current and low target impedance. In order to solve the impedance matrix of arbitrary shaped power plane, a new time-efficient complementation method, combining analytical expression with finite difference method, was proposed. Then PSO (particle swarm optimization) technique was processed to find the optimum values and positions of decoupling capacitors. Matlab simulation shows the result of the proposed method is basically consistent with that of the full-wave finite element method, but its calculation time is greatly reduced. The calculation and the measurement results both indicate that impedance of all power ports after optimization can meet the demands of their target impedance.