Accurate measurement of the thin liquid film thickness in pipes is the foundation for studying the characteristics of the film. In this paper, an interferometry-based measurement of liquid film thickness in transparent pipes is developed, which can greatly improve the accuracy, extend the lower limit of measurement and provide a new technical approach for the calibration and traceability. The light intensity distribution is established based on the optical path analysis and a mathematical model. A new algorithm to solve the direction ambiguity is developed to reconstruct the phase distribution. Besides, the effect of the pipe wall is taken into account, which can be suppressed by:image subtraction and enhancement technology. The proposed method is of high accuracy and robustness, whose reconstruction errors are 0.064% and 0.25% for the smooth and slight fluctuating liquid films, respectively.