Reinforced concrete under the longterm intrusion of seawater has been studied by the KK transform of AC impedance spectroscopy. The stability and durability of the reinforced concrete can be measured by the relative deviation of the values measured from that calculated by KK transform. With the increase of the time of the intrusion, the concrete and the rebar is less and less stable and durable. Under the same condition, concrete with fly ash is more stable than ordinary concrete.