In the petroleum industry and other fields, the thickness of the layer coated on the inner surface of containers is an important indicator to ensure the containers work in a safe or normal state. To acquire an accurate measurement, some appropriate corrections are necessary, especially for the eddy current sensor scanning over an irregular surface. The sensor’s inclining or deviating state might cause much lift-off signal to be mixed in the testing signals, which would lead to an inaccurate result. To solve this problem, this paper analyzed the influencing factors of measuring accuracy for the coating on the conical steel shells. An effective correction method was proposed based on the characteristic of testing signals. Its effectiveness for enhancing the measuring accuracy was confirmed by simulation results.