Abstract: The key technology problems such as the electrode coating and the structure design of the hydrogen concentration monitor are sovled in this paper. The on-line hydrogen concentration monitoring prototype was designed and manufactured successfully, and the measuring tests under different pressure, temperature and hydrogen concentration conditions, and the performance tests under normal conditions are conducted. The test results indicated that the above monitoring prototype could on-line monitor the hydrogen concentration with good selectivity, short response time, wide measuring range and high monitoring accuracy. The on-line hydrogen concentration monitoring devices can be applied to the HPR1000 and CAP1400 nuclear power plants.