Due to the small thickness of oil film in the actual lubrication system,conventional ultrasonic measurement methods are no longer applicable.Based on acoustic theory,the relationship between oil film thickness and acoustic reflection coefficient in multilayer media was analyzed.Two different theoretical models of oil film thickness measurement,the resonance model and the spring model were established,and the applicable range of different models was discussed.The corresponding finite element model was established for different theoretical models.The reflection coefficient of the interface was calculated by the standing wave theory and compared with the theoretical curve.It is proved that the finite element method can be used to calculate oil film reflection coefficient accurately.The results show that each model has corresponding oil film thickness measurement interval for different ultrasonic frequencies.In actual measurement,the measurement interval of each model should be determined according to the ultrasonic frequencies.