Abstract: The traditional online sodium meter of cation bed effluent has the problems of high failure rate, difficult maintenance and low input rate. Many power plants cannot judge its operation failure by monitoring the sodium ion of cation bed effluent. By measuring the hydrogen conductivity and specific conductivity of the effluent of the cation bed, the changing trend content of sodium ions can be indirectly measured,, which is applied to the on-line monitoring of the effluent of the cation bed. The conductivity meter in this method has the advantages of low failure rate, easy maintenance, convenient operation and low cost. The device can be used to judge the end point of cation bed failure, ensure the water production in its operation cycle, reduce the regeneration frequency increased by the early regeneration of cation bed, and reduce the consumption of acid, demineralized water and wastewater discharge.