The on-line measurement of steam humidity has an important effect on the safe operation of condensing steam turbine. In order to accurately calculate the experimental error of the resonant cavity affected by temperature, the theoretical analysis and experimental study of the resonant cavity temperature characteristics were carried out. Based on the research status at home and abroad, exhaust steam humidity is measured according to Maxwell equation. According to the size change of the cavity in the temperature change and the analysis of the vapor humidity data measured at different temperatures, the influence degree of temperature on the vapor humidity measured by the resonant cavity is obtained. According to the data, the conversion is about 43% steam humidity, and the normal steam turbine exhaust steam humidity is not more than 13%. The measurement error can be greatly reduced by changing the resonant cavity installation mode. The research results are of great significance to improve the on-line measurement accuracy of steam humidity.