In order to solve the problem of non-contact and non-destructive testing of lubricating oil film coated on metal substrate materials, an all-optical method was adopted to study the related problems based on the principle of intrinsic light intensity absorption. The lubricating oil film of large open gears was taken as an example, specifically, based on the intrinsic light absorption spectrum of the lubricating oil film, the optimal illumination wavelength for detection was determined. Then, the images of oil films with different thicknesses on the metal substrate were detected and analyzed, and the corresponding relationship between oil film thickness and image gray level was established. On this basis, a detection setup was built, and an optimized light source for the optimal wavelength and a charge-coupled device (CCD) were used as the illumination device and acquisition device respectively to carry out image acquisition experiments to verify the detection capability of the technology proposed in this paper. The method proposed in this paper can provide technical support for monitoring the status of large open gear lubricating oil film, and can also provide a feasible technical reference for detecting the thickness of solid adherent oil film.