Abstract: The internal lubrication condition of bearings significantly influences their operational status and lifespan, making the monitoring of the internal lubrication state crucial for ensuring the safe operation of bearings. This paper investigates the method of measuring lubricant film thickness using ultrasonic non-destructive testing technology. Initially, the ultrasonic wave propagation characteristics within the lubricant film thickness range of 1-200 μm were analyzed through finite element simulation, obtaining ultrasonic reflection signals at various lubricant film thicknesses. The study explored an ultrasonic reflection characterization method for lubricant film thickness, achieving an average relative error of 3.27% compared to the simulation's preset thicknesses within the 1-200 μm range, with satisfactory calculation accuracy within the 1-150 μm range. Furthermore, an experimental system capable of precisely controlling the lubricant film thickness was established, with a thickness control accuracy reaching 0.5 μm. Based on this, experiments on ultrasonic characterization of lubricant film thickness were conducted using an ultrasonic piezoelectric sensor with a central frequency of 10 MHz for excitation and reception to accurately measure the lubricant film thickness. The research findings indicate that the measurement results on the experimental apparatus are affected by noise and the theoretical model, with a decrease in measurement accuracy at very high or very low lubricant film thicknesses. However, for measurements within the 1 μm to 20 μm thickness range using an ultrasonic piezoelectric sensor with a central frequency of 10 MHz, the obtained results were satisfactory, at a confidence level of 0.979 2.