The catalytic effects of CuSO4 in total organic carbon (TOC) detecting processes based on supercritical water oxidation have been investigated. Using benzoic acid as a model pollutant, the presence of a CuSO4 catalyst can significantly decrease the reaction temperature and H2O2 multiple during the TOC detection processes. A better TOC conversion efficiency was obtained at a much lower temperature in the catalytic system compared with the non-catalytic condition. The use of the catalyst effectively lowered the necessary H2O2 multiple from 20.0 without catalyst to 3.0 in the catalytic system. The established device could detect the TOC concentration precisely in model wastewater without inorganic carbon (IC). Moreover, the detection of the practical wastewater was studied. Detection results were total carbon of wastewater rather than TOC of practical wastewater. A detection or removal unit of IC is necessary before it can be practically utilized.