The extinction coefficient of oil films on the sea surface was inversion using a physical model based on two-beam interference and the equal-thickness difference method. The coefficient is simplified to a quadratic equation in one variable related to oil-film thickness and incident angle and wavelength of light. Through a laboratory-simulated oil spill experiment, the reflectivities of oil films of different thicknesses were obtained. The extinction coefficients of the oil film under visible light were inversion. The model considered the light beam on the oil-film surface and effects of scattering properties and photon attenuation of the oil film on spectral reflectance.