Groundwater in the islands of the Truong Sa Archipelago is formed from rain water infiltrating into the ground, contained in rocks with high porosity and floating on the surface of salt water. Analysis of ground water quality in some islands in the Truong Sa Archipelago and assessment by Ground Water Quality Index (GWQI), contribute to forecasting environmental quality changes, or detecting sudden changes in the organic relationships between groups characteristic parameters were studied. In this article, we will evaluate the data collected on the basis of monitoring and analyzing parameters of pH, Salinity, Total dissolved solids (TDS), Ammonium (NH4+), Nitrate (NO2-), Nitrate (NO32-), Chloride (Cl-), Hardness (CaCO3). The results of monitoring and analyzing parameters show that the value ofnutritional salts (ammonium, nitrite, nitrate) is low compared to QCVN 09-MT:2015. The parameters of pH, TDS, hardness, and chloride have relatively high values. Salinity values at the islands fluctuate as follows: Truong Sa: 2,16‰ ÷ 4,12, Song Tu Tay: 2,28‰ ÷ 3,35‰, Sinh Ton: 6,80‰ ÷ 8,82‰, Son Ca: 8,17‰ ÷ 10,37‰, Nam Yet: 12,25‰ ÷ 16,15‰. Surveying the GWQI index of wells in 5 islands in the Truong Sa archipelago, basically, the wells are classified into good quality group.