Abstract: The total organic carbon(TOC) and total organic carbon ions (TOCi) of the water vapor in thermal power plants were measured and determined. Then through specific professional technical analysis, the causes of water vapor pollution can be quickly figured out. In this paper, the total organic carbon analysis technology is briefly described. Meanwhile, the water vapor pollution in three power plants is studied and the supervision of total organic carbon is suggested in response to the actual onsite situation. The results show that the abnormal increase of the hydrogen conductivity of the water vapor quality in the above three power plants is attributed to the demineralized water pollution, degradation of the cation resin and condensed water pollution respectively. Based on the analysis results, corresponding measures have been taken immediately at each power plant to resolve the problem.