In this study, sodium alginate (SA) was used as a carrier, calcium chloride (CaCl₂) was used as cross-linker to investigate the optimal conditions of immobilized chlorella and its treatment effect on ammonia nitrogen and phosphate in marine aquaculture wastewater. By comparing the effects of different concentrations of SA and CaCl₂ on the growth of Chlorella, and the effects of different immobilization conditions on the treatment of ammonia nitrogen and phosphate, the optimum immobilization conditions were 2% SA and 2% CaCl₂. The effect of immobilized and suspended Chlorella on nitrogen and phosphorus removal in simulated seawater aquaculture wastewater was compared. The results showed that the immobilized algae balls had better removal efficiency on nitrogen and phosphorus than the suspended algae. The maximum ammonia nitrogen and phosphate removal rates of immobilized algae balls at low inoculation rate (1:10) were 63.26% and 62.76%, respectively. The higher concentration of immobilized Chlorella, the stronger its purification ability. The maximum nitrogen and phosphorus removal rates of immobilized algal balls were 85.16% and 75.94% respectively according to the high inoculation rate (1:1). The average removal rates of ammonia nitrogen and phosphate under continuous flow operation were 84.49% and 72.17%, respectively. The immobilized state of Chlorella retained and extended the suspended growth activity, and enhanced the efficiency of nitrogen and phosphorus removal from seawater aquaculture wastewater.