Abstract: According to the 10-year monitoring data of pH in Sanya Bay, the ranges of pH in the surface and bottom water are 8.01~8.23 and 8.01~8.24, respectively, with 8.14 on average. The pH has showed a decreasing trend since 2001, decreasing from 8.30。to 8.07 on annual average during 2001~2010, which is greater than the global ocean acid velocity. Four types of vertical distributions of pH are found in Sanya Bay: 1) Upper layer is smaller while bottom layer is greater; 2) Upper layer and bottom layer are greater while middle layer is smaller; 3) Upper layer and bottom layer are smaller while middle layer is greater; 4) Upper layer, middle layer and bottom layer reveal an obvious decreasing trend. From the aspect of horizontal distribution, the pH varies greatly in different seasons and lower pH is mainly found in the estuary. By analyzing the effects of pH on coral reef, we find that when the pH increased from 7 to 8, the content of dissolved calcium decreased from 12 mgL-1 to 1 mgL-1. Therefore, the pH in Sanya Bay is currently still within the range of normal growth of coral reef (8.0~8.3), and no negative effects are found.