The effect of relative humidity on the corrosion-resisting property of fluorocarbon coating was investigated by water vapor transmission technique and electrochemical impedance spectroscopy technique. Measurements were carried out on samples, which were tested in an accelerated corrosion environment for the same time (300, 500, 800 and 1000 h). The experimental results were obtained as follows: (i) The water absorption, coating resistance and corrosion rate of metal under the coating changed with relative humidity and aging time, the results obtained by EIS are good consistent with those by water vapor transmission technique; (ii) when the subject investigated presented two time constant, the goodness of fit by EEC R(C(R(CR))) was inferior to that by EEC R(C(R(QR))), but the error of parameters acquired from the former was smaller, by which we could analyze the experiment result quantitatively. With the appearance of diffusion layer on the metal, the difference of metal capacitance was aggravated, the error of parameters acquired from EEC R(C(R(C(RW))))was bigger than that from EEC R(C(R(Q(RW)))).