In order to solve the problem that the water film and salt scale on the inner wall of the resonant cavity sensor could reduce the accuracy measurement of the wetness, a method for measuring the dielectric film thickness on the inner wall of cylindrical cavity is proposed in this paper. Based on the principle of dielectric perturbation method, this method uses the hyperbolic cosine-shaped slot resonator as a sensor, working at the TE111 mode to measure the dielectric film thickness. Taking the water film as an example, the relationship between the water film thickness and the resonant frequency is deduced theoretically, and the feasibility of measuring the water film by TE111 mode is analyzed through simulation. We establish the mathematical model of the current density distribution line of the cavity end face for the TE111 mode by the method of vector field equation, and design the hyperbolic cosine-shaped slot resonator sensor. The simulation results show that the performances of the electromagnetic characteristics, radiation characteristics and flow characteristics of the resonator are all relatively well. The electromagnetic leakage rate is basically 0, and the sampling error is -1.25%, which meet the design requirements. The water film thickness, salt scale, oil film, fouling and other dielectric films can be measured accurately. It is also shown that the film thickness is linearly related to the resonance frequency offset, which are consistent with the theory.