Pulsed eddy current testing has attracted increasing attention on ferromagnetic materials evaluation as it has abundant time–frequency domain features that can be utilized to characterize the thickness of the materials. Few studies have given a theoretical analysis to explain the relationship between the features and the thickness. In this article, the decay process of eddy current response to a ferromagnetic casing is analyzed qualitatively, and a concise expression of the decay rate is derived. The expression reveals the relations between the decay rate and the casings parameters. A method based on the decay rate, thus, is given for the wall thinning evaluation. Both numerical and practical experiments are carried out for verifying the method, and the experiment results demonstrate that the method is barely influenced by testing coils and the inner radius of a ferromagnetic casing. The wall thinning of a ferromagnetic casing can be evaluated appropriately.