Abstract: Lubrication film thickness measurement based on ultrasonic technology has been preliminarily verified in the field of engineering, but there is still a lack of research on dynamic lubrication film thickness measurement. There is a blind area in the measurement of film thickness between the spring model and the resonance model based on ultrasonic technology. This is because the traditional model only uses the amplitude of reflection coefficient, leading to a certain blind area in the measurement range of the film thickness between the two models. In order to make full use of the information of the phase and amplitude of the reflection coefficient, a composite model is proposed to cover the blind area in the measurement. In order to realize the on-line monitoring of film thickness, a real-time ultrasonic measurement system of dynamic film thickness was built, and the influence of pair type and lubricant type on measuring blind area was studied. The theoretical model of ultrasonic measurement was established and the propagation law of ultrasonic wave in different friction pairs and different lubricants was simulated and analyzed. The results showed that the self-established measurement system could accurately track the film thickness with different periods and different waveforms. The film thickness with rectangular waveform had the best tracking accuracy, followed by sine waveform and triangle waveform, the sawtooth waveform had the worst tracking accuracy, and the tracking accuracy increased gradually with the increase of the period. It was found that one of the important sources of measurement error was the resolution of oscilloscope. Due to the resolution limitation, there was a gap between the actual measured resonance frequency and the ideal resonance frequency, so there was an error between the measured film thickness and the theoretical film thickness. The proposed composite model can measure a wide range of film thickness without blind area by a single probe. With the increase of the acoustic impedance of the lower friction pair and lubricant, the measuring blind area of film thickness decreased gradually. The larger the acoustic impedance of the lower friction pair was, the larger the reflection coefficient of the lubricant-lower friction pair interface was, the smaller the transmitted acoustic pressure was, and the reflected sound wave containing the film thickness information was enhanced. Therefore, the measurement blind area was inversely proportional to the acoustic impedance of the lower friction pair. The reflection coefficient of the upper friction pair-lubricant interface increases with the decrease of the acoustic impedance of the lubricant, that was, the energy of the reflected wave b₁ increased, the energy of the transmitted wave entering the lubricant decreased, so the energy of the reflected wave b₂ containing the film thickness information also decreased. Therefore, the measurement blind area increased with the decrease of the acoustic impedance of the lubricant, and the measurement range of the film thickness of the three models became smaller. When the spring model is used to measure large film thickness, the amplitude curves of reflection coefficient overlap each other and difficult to distinguish. When the acoustic impedance of the lubricant is too small, "unmeasurable vacuum zone" can easily occur. This paper provides theoretical and experimental data for on-line monitoring of lubrication film thickness of precision rotating parts under actual working conditions.