Abstract: In recent years, coral bleaching events have frequently occurred globally due to global warming and other factors. However, the Coral Reef Watch (CRW) program established by the National Oceanic and Atmospheric Administration (NOAA) has underestimated the actual situation of coral bleaching in the South China Sea. Based on 180 cases of coral bleaching in the South China Sea and its surrounding waters since 1985, this paper obtains the optimum threshold combination by calculating the false negative rate (FNR), the false positive rate (FPR), and the accuracy (ACC) of different threshold combinations, thus improving the detection accuracy of coral bleaching events in the South China Sea. The results are as follows: (1) The FNR of the bleaching detecting results obtained using the NOAA threshold was 70.70%, indicating the long-term underestimation of the coral bleaching; (2) With the optimized critical threshold (CT) and alert threshold (AT), the ACC was improved from 58.13% to 73.90%, meanwhile the FNR and FPR were both less than 30%. As revealed by the coral bleaching event in the Nansha Islands in June 2007, the optimized thermal stress index can be used to effectively detect the event and mark the bleaching alarm level in time compared to the past underestimation. Therefore, the improved method for thermal threshold detection can improve the monitoring level of coral bleaching and are conducive to the management and protection of coral reefs in the South China Sea.