Abstract:A coral reef ecosystem has important functions. It can provide marine life with habitats, protect coastlines from storm damage, erosion and flooding, and provide resources for tourism, construction materials, pharmaceuticals and other chemicals. But with the rise of global climate temperature and CO2 concentration, coral bleaching has become more frequent and coral reef systems face more challenges. Factors which can cause coral bleaching include abnormal sea water temperature, solar and ultraviolet radiation, reduced salinity, coral disease, marine pollution, Crown-of-Thorns Starfish, over-fishing of marine species and the rise of global CO2 concentration. Abnormal high Sea surface temperature (SST) is the key factor causing coral bleaching. Corals can adapt to environmental changes by two mechanisms: physiological acclimatization and exchange of symbiotic algae. The mechanism of physiological acclimatization involves the xanthophyll cycle, fluorescent coral pigments, oxidative enzymes, MAAs and heat shock proteins (Hsps). The mechanism of exchange of symbiotic algae is the Adaptive Bleaching Hypothesis, which is still disputed and needs more evidence. Future research work should pay more attention on coral holobiont, especially on the contribution of host factors to symbiosis. The Australian coral scientist Ove Hoegh-Guldberg proposes that corals and their zooxanthellae will be unable to acclimate or adapt fast enough to keep pace with the present rapid rate of warming of tropical oceans. Some climate models still project that, over the next 50 years, temperature increases will exceed the temperature conditions under which coral reefs have flourished over the past half million. The future of coral reesf is not exciting.