Based on the surface plasmon resonance (SPR) effect caused by a modified Otto structure of cylindrical lens, a method for measuring the thickness of metallic thin films is proposed. For this proposed method, the background intensity can be fitted only with the single SPR absorption image obtained under the p-polarized light incidence, no need using the s-polarized light. Hence, the normalized reflectivity curve in the vertical direction is obtained. The reflectivity curve is fitted by establishing a model of the optical thin film and the thickness parameters of the metallic thin film are derived. In the experiment, a sample of Au film with a thickness in the nanometer range is measured. The measurement results show that the difference between the measured average thickness of Au film and the result obtained by commercial spectroscopic ellipsometer is only 0.1 nm, which verifies the effectiveness of this method.