Abstract: Separated by Qiongzhou Strait, Leizhou Peninsula and Hainan Island have similar climate and geological environment. Recently,with the quick development of economy and society, the extract and usage of underground water is becoming frequent and the problems of development against environment protection have become prominent. In order to understand the distribution, variation of underground water and the link between distribution, variation and environment, the authors drilled shallow underground monitoring wells, and then sampled and analyzed the samples in laboratory along Leizhou Peninsula and Hainan Island shorelines. The analysis results show that beside few wells, the concentration of total amount of dissolved solids (TDS) and the main ions (Cl^-, S$\begin{array}{c}{O}_4^{2-}\end{array}$ and Na⁺) in underground water are at high level. TDS and Cl^- have maximum concentration nearly to 39.00 g/L and 20.00 g/L respectively, and also are featured with relatively frequent vibration, revealing that the type of underground water changed from weak acidic and bicarbonate to Cl^- types and are related with tidal cycles. Constrained by geological environment, the resilience of shallow underground in most wells falls into average or weak grade and the water quality is IV-V types, which will not change at least in shortterm. Therefore, the underground water quality has close relationship with geological environment and human activities, and it is probably affected by the release of deep ancient seawater. The influence factors of the underground water quality are complicated, so more attention should be paid to the further monitoring and research.