Through the 26 monitoring sections established in Haikou, 7 water quality indicators were recorded. GIS spatial analysis functions were used to conduct water quality analysis and map water quality variations in Haikou. The results showed that the water quality was worse in non-flood seasons than during flood seasons. The primary pollutant is NH₃-N, whose average value (4.44mg·L^-1) exceeds the Class V water quality standard and has a high variation coefficient (177%). There are significant spatial differences, which are greatly affected by human activities and the tertiary industry. The primary pollutant in the flood season is TP, with an average value of 0.47 mg·L^-1, a strong variation with significant spatial differences. From the perspective of spatial distribution, the overall water quality shows a trend of deterioration from inland to near the sea entrance. The most severe pollution is the H2 cluster, which is located in the main urban area and where many rivers enter the sea. It is a key area for water pollution prevention and control.