Abstract: The cylindrical resonant cavity in steam turbine, as a wetness measurement sensor, works in TE₀₁₁ mode. Multiple annular slots are grooved at different positions on both ends of the cylindrical cavity to sample the wet steam in real time. The electromagnetic field radiates through these slots to the surroundings, influencing the resonant characteristics and electromagnetic environment of the resonant cavity. Research on the radiation from the annular slots at both ends of the cylindrical resonant cavity is valuable to reduce electromagnetic radiation. The electromagnetic field equivalence theorem, boundary value condition and Hankel transform were applied. The electromagnetic field distribution of the slots structure was derived by mode matching, and the radiation situations of different number, position, width, and thickness of annular were given. MATLAB was used to calculate the radiation of the annular slots, and HFSS simulation software was used to simulate the cylindrical resonant cavity with annular slots, and numerical results of radiation of the annular slots were obtained. The simulation results verify the correctness of the calculation method. The optimum structure parameters of cylindrical resonant cavity with annular slots as the wetness sensor is presented.