The uniform numerical method is proposed to analyze the transient performances of grounding system with grounded secondary cable. The influences of different factors on the electromagnetic interferences applied on the secondary systems due to the lightning current invading into the substation, including materials of grounding electrodes, lightning current wave-shape, and the parameters of secondary cables. The length of the secondary cable has strong influence on the transient potential difference between the core and the shielding sheath of secondary cable.