Abstract: Ultrasound has been applied to measure the lubricant film thickness between the piston assembly and cylinder wall in recent years. However, when the lubricant film thicknesses vary in a wide range, there is a blind zone between the resonant model method and spring model method, which greatly limits the application of ultrasound. In this paper, the lubricant film thickness measurement method based on the ultrasonic reflection coefficient phase shift was studied to overcome the limitation of two conventional methods. The principle of the ultrasonic reflection coefficient phase shift method was analyzed, then an accurate method to solve lubricant film thicknesses directly was proposed for the first time, with the condition that the acoustic impedance of media on both sides of the lubricant film were equal. A lubricant film thickness measurement system was used to measure the film thicknesses from 10 to 90 μm by single center frequency. The measured results were in good agreement with the set thicknesses, which demonstrated that the lubricant film thickness measurement method based on the ultrasonic reflection coefficient phase shift can measure the lubricant film thicknesses in a wide range.