Applying the theory of capacitance measurement and capacitor working principle, a kind of capacitive steam humidity measurement system has been developed, which includes steam humidity adjusting system, external measuring circuit and steam humidity calibration system. Use this steam humidity measurement system; a kind of coaxial cylindrical capacitance sensor is studied. The results shows that, the coaxial cylindrical capacitance sensor has stable static characteristics; capacitance sensor output characteristic does not change with the steam flow; the output capacitance of the sensor increases with the steam humidity, and they has a good linear relationship.