To investigate the threshold of purification efficiency of Ulva meridionalis for aquaculture wastewater，in addition to identifying the optimal algal density and treatment duration for the purification of such wastewater，U. meridionalis was cultured in simulated aquaculture wastewater using a bluepard biochemical incubator. Six density gradients of 0.5，1.0，1.5，2.0，2.5g/L（treatment groups）and 0 g/L （control group）were established. The concentration of NH⁺₄-N、NO^-₂-N、NO^-₃-N and PO^3-₄-P in the simulated aquaculture wastewater was respectively measured at 0，12，24，36，48，60，and 72 hours. U. meridionalis demonstrated a significant purification effect on aquaculture wastewater （P＜0. 05）. The optimal U. meridionalis density for effective purification of the culture wastewater was determined to be 2. 5 g/L. The optimal treatment times for achieving effective purification of NH⁺₄-N、NO^-₂-N、NO^-₃-N and PO^-₄-P in aquaculture wastewater，at the optimum algal density，were found to be 24，60，48，and 72 hours， respectively. Additionally，the removal efficiency of U. meridionalis for different forms of inorganic nitrogen was influenced by the composition of nitrogen sources present in the aquaculture wastewater. It was observed that NH⁺₄-N was preferentially absorbed，followed by NO^-₃-N，and finally NO^-₂-N. The treatment of aquaculture wastewater with U. meridionalis s at an algal density of 2. 5 g/L for a duration of 72 hours yielded the most effective purification of nitrogen and phosphorus in the wastewater. This study provided data support for biological purification of aquaculture wastewater.