The observations of the increasing greenhouse gases driving the global warming over the past century shows that the warming will continue in the future. In this study, the trend of sea surface temperature (SST) over offshore China in the 21st century is assessed based on the fifth phase of the Coupled Model Inter Comparison Project (CMIP5). The results show that under the midrange mitigation emissions scenario (RCP4.5), SST over most of offshore China will increase by nearly 1℃around the year 2030 relative to the historical period (1970~2005). SST in the middle and high latitudes sea areas will rise faster than that of low latitude. Specifically, the East China Seas (including Bohai sea, Yellow sea and East China sea) will increase by about 1℃, 2℃, and 3℃ between the period 2030~2039,2060~2069, and 2090~2099, respectively, the magnitude of which are larger than that in the South China Sea. Under the doubled CO_2 emissions scenario (RCP8.5), the Bohai sea SST will rise up by about 5℃ by the end of the 21st century, and it will be the biggest warming areas in the world. These changes will pose serious risks to marine ecosystems and society on and around offshore China. It is an impress issue to curb the greenhouse gas discharge and bring down the risks of climatic changes.