Abstract: Based on relations for wetness measurement by microwave resonant cavity, the sensitivity coefficients of relevant relations were deduced, and the uncertainty of direct measurement parameters was analyzed, such as pressure and frequency, etc. Taking the steam of different pressure and wetness as an example, the uncertainty of various calculation parameters in the wetness measurement relations was studied, while standard uncertainty of the wetness measurement system was combined, and the influence of both steam pressure and wetness on the system uncertainty was analyzed. The comprehensive system uncertainty was identified, considering parameters measurement, cavity heat expansion, sampling error, sedimentary water film on inner surface of cavity and so on. Results show that the uncertainty of the wetness measurement system is less than or equal to 0.764%, and its combined standard uncertainty increases with the rise of steam pressure and wetness. The system uncertainty is slightly affected by pressure measurement, but greatly influenced by frequency measurement. Therefore to guarantee the precision of a wetness measurement system, it is important to measure the resonant frequency accurately.