In this paper the inhibiting function and its mechanism of the Ce³⁺ion to the pitting corrosion on LY12CZ aluminium alloy in O.1mol? L^-1 NaCl solution are studied by using coulostatic pertubation method. The results of research show that the process of the pitting induction and the formation of cerium conversion film on aluminium alloy surface have similar electrochemical model. The rate controlling step of the electrochemical corrosionis thei nterface process of passivation layer(or conversion film)and solution. The Ce³⁺ can inhibit the pitting induction and developing process of the alluminium alloy. After the cerium conversion film is formed，the electrical impedance on the surface of aluminium alloy would increase and the anticorrosion will enhance.