The single and synergy effects of Ulva lactuca and Sesuvium portulacastrum on the purification of mariculture wastewater were evaluated. The results showed that the water quality was significantly improved by U. lactuca with different planting densities after 12 days of experiment. The density group of 2.0 g·L-1 showed the highest removal rates of COD, inorganic phosphorus, ammonia nitrogen and total nitrogen, which were 41.7% 76.9%, 81.8%, 67.2%, respectively. S. portulacastrum of different planting densities also improved the water quality significantly after 12 days of treatment. The planting density of 4.8 g·L-1 had the highest removal rates of COD, inorganic phosphorus, ammonia nitrogen and total nitrogen, which were 49.0% 20.8%, 98.5%, 55.9%, respectively. In the U. lactuca and S. portulacastrum synergy group, the removal rates of COD, inorganic phosphorus, ammonia nitrogen and total nitrogen reached 36.7%, 100%, 100.0% and 88.8%, respectively. The significant removal efficiencies of U. lactuca and S. portulacastrum on nitrogen and phosphorus nutrients indicated that the purification ability on mariculture water can be enhanced by utilizing mixed aquatic plants. Our results indicated that U. lactuca and S. portulacastrum can both improve the water quality, and appropriate planting density and suitable collaboration of plants significantly promote the phytoremediation effects.