Ｕ₃₇^Ｋ is a ratio index of long-chain unsaturated alkenone，synthesized by a group of prymnesiophyte algae，most notably the coccolithophorid Emiliania huxleyi，which is ubiquitous in pelagic marine sediments from quaternary to cretaceous period. Ｕ₃₇^Ｋhas a intimate relation with planktonic foraminifera's 8180 in marine sediments. Laboratory culture，marine particulate，and sediment core top studies have indicated that Ｕ₃₇^Ｋis closely related to sea surface temperature. so it is regarded as an index of paleotemperature in paleoclimatic studies and has been applied to estimate SST in cores from organic-rich depositional environments at low latitudes，which can also compensate for the foraminifera's δ¹⁸O defects in calculating paleotemperatures in marine sediments under the CCD.
    This paper mainly introduces the theory of SST index:Ｕ₃₇^Ｋ,the Ｕ₃₇^Ｋ-T °C equation and their researches and problems in current studies.