Abstract: For the necessity of monitoring mechanical structure operation,the thickness measurement of thin fluid layer in mechanical structure by using ultrasonic technique is theoretically studied.A model of three layers of media is constructed to simulate the propagation of normal incidence ultrasound.Considered the continuous boundary conditions,the reflection coefficient of thin fluid layer to normal incidence longitudinal waves is obtained from the continuous model.By study the reflection coefficients of different thicknesses of fluid layers,two models,resonant model and spring model,are developed to characterize the thickness of fluid layer respectively by the resonant frequency and stiffness coefficient.Meanwhile,the influence of acoustic properties of three layers on the reflection coefficient and the measurement of thickness is studied.