Accurate measurement of steam humidity is of great significance for maintaining the stability of steam turbine. The measurement of steam humidity is affected by the thickness of the water film. To solve the problem of measuring the thickness of water film on the inner wall of cylindrical cavity, this paper proposed a new method for measuring the thickness of dielectric film on the inner wall of cylindrical cavity based on the principle that the resonant frequency of a dielectric loaded cylindrical cavity varies with the thickness of dielectric film. The characteristic equation of cylindrical waveguide filled with double-layer dielectric was derived according to Maxwell's equations and boundary conditions of dielectric loaded cylindrical waveguide. Under the given dielectric film thickness, the relationship between the phase constant of propagation mode and the working frequency of waveguide was given according to the characteristic equation. Based on the derived characteristic equation, the study established the model of relationship between the thickness of dielectric film on the inner wall of cylindrical cavity and the value of resonance frequency. The resonant frequencies corresponding to different media and different thicknesses were obtained by numerical solution with MATLAB software. Under the same parameters as those set by numerical solution, the resonant frequencies corresponding to the cylindrical cavity loaded with double-layer media were simulated and analyzed by HFSS software. Comparing the simulation data with the theoretical calculation data, the results show that with the increase of the dielectric film thickness, both of them show a parallel downward trend, and the resonance frequency offset is linearly related to the dielectric film thickness, which verifies the feasibility of this method. There is no limit to the thickness of dielectric film measured by this method (within radius), and this method is universal. A network analyzer was used to carry out related experiments, and a practical measurement system was built. When the thickness of the medium is within 100 μm, every 10 μm interval, the interval of the measured medium thickness value is about 6.8 μm ~ 7.1 μm, and the relative measurement error does not exceed 24.6%. The result of comparison between theoretical data and experimental data shows that the data measured by this method is more accurate.