Abstract: To clarify the water quality characteristics of pond water during the stage of tilapia (Oreochromis) reaching the harvestable size in Wenchang, Hainan and provide guidance for aquaculture tailwater treatment, this study collected water samples from 21 ponds (7 per model) under three main culture modes in the main tilapia farming area of Wenchang, Hainan: low-density non-water-exchange (LNC), low-density water-exchange (LC), and high-density non-water-exchange (HNC). Total nitrogen (TN), total phosphorus (TP), permanganate index (COD_Mn), and total suspended solids (TSS) were detected, and water quality assessment was performed. Results showed that water quality of all 21 ponds exceeded the Grade II limits of the 《Discharge standard of water from aquaculture》, with the pollution severity of indicators ranked as TN > TP > COD_Mn > TSS. Among the three modes, HNC had a higher water pollution level than the other two. Both surface and bottom water exceeded the Grade II limits; TSS and COD_Mn in bottom water were significantly higher than those in surface water (P<0.05), while no significant differences in TN and TP were found between the two layers. In summary, pond water during the stage of tilapia reaching the harvestable size under all three modes in Wenchang, Hainan is heavily polluted, with nitrogen and phosphorus as the main pollutants. Both surface and bottom water require treatment to meet the discharge standards. This study provides data and a theoretical basis for tilapia aquaculture tailwater discharge and treatment in Hainan.