In order to measure lubricant film thickness of sliding bearing accurately, a fiber-based spectral-domain optical coherence tomography (SD-OCT) detection system was constructed. The detection system could perform high-resolution imaging to lubricant film with SD-OCT, and the lubricant film thickness was determined according to the relative location of lubricant film and bearing surface in one-dimensional depth image and two-dimensional tomography image. The measuring principle of the SD-OCT was detailed and performed in a film thickness measuring experiment, in which the interference spectrum decouple method was used to reduce the effect of noise on measurement results. Experimental result shows that the system with the measuring error lower than 2 μm exhibits good repeatability and reliability, thus realizing rapid and accurate measurement of lubricant film. The system is expected to be applied in on-line monitoring of bearing operation of mechanical equipment.