The research studies the water quality of O Loan Lagoon, which has been active in aquaculture in recent years. Based on the water quality index (WQI) calculations of 15 water samples collected in 3 periods from September 2020 to October 2020, spatial and temporary polluted distribution maps were established using ArcGIS software. The results indicate that the WQI value ranges from 1 to 80, reaching an average of 58. This estimation shows that the water quality of O Loan Lagoon is only average during the rainy season in 2020. Water quality distribution maps show that the southern region's water quality is poorer than the rest. This result demonstrates the negative impacts of aquaculture activities on the coastal lagoon's water environment, and local authorities need appropriate solutions for sustainable aquaculture activities in this area.