Abstract: The paper describes a system concerning the leakage of nuclear power station primary loop. Thesensor of leak--detection system is a special humidity--sensitive element. In this system,the collection andtreatment of detected data,and the alarm of leakage are controlled by a simple board computer. Thethree parameters A, B and C are set up in this system, and the alarm will give out when signal surpassesvalues of B and C at the same time. The effects of environment humidity and temperature on collectionof detected data are considered, and the alarm of leakage is reliable. When the leakage rate of water at320℃ and 12. 2 MPa is 0.3g/min, the alarm signal will sent out within 5s.