In orderto accurately measure the relative humidity of high-temperature and high-pressure steam with a dew pointtemperature above 100 ℃ , a relative humidity measurement system based on the condensation method has been designed, with the main core components including a condensation weighing device, a non-condensable gas humidity measurementdevice and a data processing system. Based on the relationship between steam relative humidity and moisture content, the measurement system realizes the online measurementof steam relative humidity value under the temperature and pressure conditions of the target steam by real-time monitoring of condensate quality and non- condensable gas moisture content. The condensation method can effectively measure the relative humidity of the gas in the range of 0.15% ~ 100% under the condition of dew point temperature of - 40 ℃ ~ 130 ℃ . The measurement uncertainty ofthe condensation method is0.59% atthe steam dew pointtemperatureof128℃ and therelativehumidity of 95% , and the relative humidity measurement deviation is less than 0.4%. It has the characteristics of high measurementaccuracy and fast measurement rate. This system can realize the instantaneous measurement and can realize the traceability ofthe internationalsystem ofunits(SI) ofthe relative humidity value.