Microplastics (MPs) pollution had become a major global environmental issue, and the combined toxic effects of MPs and other environmental pollutants in the ocean had gradually attracted attention. Acanthopagrus schlegelii was an important aquaculture species in the East China Sea. To explore the combined effects of environmental microplastics and ammonia nitrogen on its juveniles, this study employed low-concentration (A1) and high-concentration (A2) ammonia nitrogen alone, as well as low-concentration (A1M1) and high-concentration (A2M2) ammonia nitrogen combined with microplastics for exposure treatments. The changes in growth, antioxidant, and immune indices of juveniles in each group were compared. The results showed that the weight gain rate and specific growth rate of A. schlegelii juveniles decreased significantly with increasing concentrations of ammonia nitrogen and microplastics in the treatment groups (P<0.05), while the survival rate showed no significant difference from the control group (P>0.05). The activity of superoxide dismutase (SOD) in the liver of groups A1 and A2 was significantly lower than that in the control group (P<0.05), and the SOD activity in group A1M1 was significantly higher than that in the control group (P<0.05). However, there was no significant difference in catalase (CAT) activity between any treatment group and the control group (P>0.05). The activity of aspartate aminotransferase (GOT) in the liver of groups A1, A2, and A1M1 was significantly lower than that in the control group (P<0.05). The activity of alanine aminotransferase (GPT) in group A2 was significantly lower than that in the control group (P<0.05), while the GPT activity in group A2M2 was significantly higher than that in the control group (P<0.05). In addition, the activity of lysozyme (LZM) in the liver of groups A2 and A2M2 was significantly lower than that in the control group (P<0.05). The results indicated that single ammonia nitrogen exposure significantly reduced the activities of superoxide dismutase and lysozyme in the liver of A. schlegelii juveniles. In the combined exposure group of ammonia nitrogen and microplastics, the activities of aspartate transaminase and lysozyme decreased significantly, while changes in antioxidant enzymes were not significant. These findings suggested that both treatment methods affected the metabolism, antioxidant capacity, and immunity of A. schlegelii juveniles, thereby inhibiting the growth and health of the organisms. Moreover, microplastics exhibited a combined effect on ammonia nitrogen stress, which further influenced the antioxidant and metabolic capacities of juveniles in the combined treatment group. The findings of this study helped deepen the understanding of the potential environmental toxicity of microplastics, provided a reference for the combined effects of microplastics and other pollutants in actual ecosystems, and offered a theoretical basis for the objective assessment of marine ecological risks posed by microplastics and other pollutants.