A sensitivity-improved reflection coefficient amplitude method for determining wide-range lubricating oil film thickness is proposed based on partial reflection waves, which are obtained by removing the first reflection wave. Numerical analysis and experimental testing are conducted to verify the proposed method. The results show the lubricating oil film thickness ranging from the spring model region to the resonant region can be calculated based on the proposed partial-wave reflection coefficient amplitude method. In comparison to the existing reflection coefficient amplitude method, the measurement accuracy of the lubricating oil film thickness is significantly improved for the transition region which locates between the spring model region and the resonant region, for instance, the observed maximum relative error is decreased to 9% from 60%.