Coral is a good medium for the study of high resolution climate changes in tropical oceans. In this paper, we analyzed the C, O, B isotopic compositions of 49 samples from a living Porites lutea coral, SY10, from Sanya Bay, south of Hainan Island. The result shows that, the delta13C, delta~(18)O and delta~(11)B values of the SY10 coral range from -3.32 to -1.76, from -6.13 to -4.78 and from 23.51 to 26.23, respectively. They all exhibited obvious seasonal fluctuations. A significant positive correlation occurs between the reconstructed seawater pH and delta~(18)O, which is converse to the temperature effect for the solubility of CO_2 in seawater that low temperature favors the dissolution of CO_2 and hence decreases seawater pH. Our observation that higher pH values tend to appear in low temperature seasons suggests that seawater pH on coral reefs should be largely controlled by biological activities rather than changes in CO_2 solubility. Meanwhile, the reconstructed seawater pH values range from 7.77 to 8.37, with seasonal fluctuations similar to that of the delta~(11)B. Furthermore, the substantial periodic fluctuations of reconstructed pH are in consistent with the variability of observed seawater pH records on the same coral reef, and agree with the seawater pH changes in Xisha Islands mentioned by Dai et al. (2009). We therefore conclude that it is feasible to reconstruct high resolution seawater pH by using boron isotopic composition of corals.