As an important alternative to alleviate the shortage of wild fishery resources, mariculture is facing increasing challenges on the wastewater treatment, mainly due to the salinity brought from seawater and low nutrient concentration. In this study, Spirulina platensis (S. platensis) was adopted as the target algae stain for synthetic mariculture wastewater treatment, which exhibited excellent adaptability to high-saline wastewater during a 40 days’ adaptive culture. Then, the microalgae stain was inoculated into photo-bioreactors with different uplift airflow velocity (UAV) to achieve microalgal aggregation. After 10 days of pre-cultivation and a 30-day granulation process, the maximum nutrient removal rates by S. platensis were 86.5% of TN (to 3.4 mg/L), 98.1% of TP (to 0.1 mg/L) and 95.8% of DOC (to 5.5 mg/L), with 3.5 g/L of biomass content in synthetic wastewater. Analysis of phosphorus and EPS content showed that higher protein content in tightly bound EPS and enhanced P accumulation was accompanied with the microalgal aggregation processes, and P was mainly distributed in the residual rather than the EPS part of microalgal aggregates, indicating that the development of aggregates from suspension S. platensis would benefit not only biomass separation, but also phosphorus recovery, being a potential treatment for simultaneously removing nutrients and recovering mariculture wastewater.