In this paper, we propose the use of the modified nodal analysis (MNA) method to rederive cavity-mode equation of PCB power-bus structures with discrete capacitors. In some frequency, it shows impedance inaccuracies in the traditional double summation to calculate the transfer impedances for the studied PCB power-bus structures with discrete capacitors. Using our proposed method, we not only achieve the accuracy superiority but also accomplish the computational efficiency. The theoretical results are well confirmed by measurement and electromagnetic simulation software tool.