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To measure the steam wetness in a steam turbine accurately, the measurement error caused by the deposited water film on the inner wall of the wetness sensor should be deducted. A hyperbolic slot-shaped resonant cavity sensor for measuring the thickness of the liquid film on the inner wall of a cylindrical cavity is designed in this paper. We establish the relationship between the liquid film thickness and resonant frequency theoretically by the medium perturbation method and verify the feasibility of measuring the liquid film thickness in the TE111 mode. The mathematical model of the current density distribution line at the end of the cavity in the TE111 mode is deduced by the method of the vector field equation. Based on the mathematical model, the sensor is designed by Full-wave simulation software. The simulation results show that the electromagnetic leakage rate of the sensor is basically 0% and the sampling error is no more than -1.3%. The liquid film thickness has a linear relationship with the frequency offsets. The experimental results show that it is feasible to accurately measure the liquid film thickness on the inner wall of a cylindrical cavity by measuring the resonant frequency of the resonator. When the thicknesses of the liquid film are within 100 um, the measuring error does not exceed 0.245, which is about 50% lower than the conductivity method.