<p>Tridacna is the largest marine bivalves in the tropical ocean, and its carbonate shell can shed light on high-resolution paleoclimate reconstruction. In this contribution, δ<sup>18</sup>O<sub>shell</sub> was used to estimate the climatic variation in the Xisha Islands of the South China Sea. We first evaluate the sea surface temperature (SST) and sea surface salinity (SSS) influence on modern rehandled monthly (r-monthly) resolution <i>Tridacna gigas</i> δ<sup>18</sup>O<sub>shell</sub>. The obtained results reveal that δ<sup>18</sup>O<sub>shell</sub> seasonal variation is mainly controlled by SST and appear insensitive to local SSS change. Thus, the δ<sup>18</sup>O of <i>Tridacna</i> shells can be roughly used as a proxy of the local SST: a 1 ‰ δ<sup>18</sup>O<sub>shell</sub> change is roughly equal to 4.41 °C of SST. R-monthly δ<sup>18</sup>O of a 40-year <i>Tridacna squamosa</i> (3673 ± 28 BP) from the North Reef of Xisha Islands was analyzed and compared with the modern specimen. The difference between the average δ<sup>18</sup>O of fossil <i>Tridacna</i> shell (δ<sup>18</sup>O = −1.34 ‰) and modern <i>Tridacna</i> specimen (δ<sup>18</sup>O = −1.15 ‰) probably implies a warm climate with roughly 0.84°C higher in 3700 years ago. The seasonal variation in 3700 years ago was slightly decreased compared with that suggested by the instrument data, and the switching between warm and cold-seasons was rapid. Higher amplitude in r-monthly and r-annual reconstructed SST anomalies implies an enhanced climate variability in this past warm period. Investigation of the El Ninõ-Southern Oscillation (ENSO) variation (based on the reconstructed SST series) indicates a reduced ENSO frequency but more extreme El Ninõ events in 3700 years ago.</p>