This study was designed to investigate the effects of dietary lysozyme supplementation on serum biochemical indices, antibacterial properties and liver antioxidant capacity of GIFT tilapia (Oreochromis niloticus) under ammonia-nitrogen stress. Juvenile tilapia with the initial weight of (11.35±0.08) g were fed with six experimental diets (labeled as L0, L18, L36, L54, L72 and L90) which were supplemented with graded lysozyme levels (0, 18, 36, 54, 72 and 90 mg/kg) in basal diet for 60 days. After that, a 24 h ammonia-nitrogen stress test with ammonia chloride was conducted to the experimental fish above. The results showed that: ① After stress, fish serum biochemical indices presented a significant difference and the fish showed different feedback responses among groups (P < 0.05). The fish in L54 group could reduce damage from ammonia-nitrogen stress through stimulating the immune system and regulating protein metabolism; while mainly through regulating the dynamic variation between high/low density lipoprotein cholesterol, cholesterol and triglyceride in L72 and L90 groups. ② Serum antibacterial test showed that there was the maximum resistance ability to Escherichia coli, Staphylococcus aureus, Aeromonas hydrophila in L54 and L72 groups and higher inhibiting ability to Vibrio alginolyticus in L36~L90 groups (P < 0.05), while lysozyme supplementation was helpful for the proliferation of Bacillus subtilis (P < 0.05). ③ Liver superoxide dismutase, catalase and glutathione peroxidase activity after ammonia-nitrogen stress were generally enhanced with increased dietary lysozyme level and the highest value was presented in L54 group (P < 0.05); Malonaldehyde content in each lysozyme supplemented group was significantly lower than the control group (P < 0.05). In summary, our results indicate that ingestion of a basal diet supplemented with 54 and 72 mg/kg lysozyme could produce a most positive and effective regulatory response on serum biochemical indices, antibacterial properties and liver antioxidant indices under ammonia-nitrogen stress, so as to improve fish anti-stress ability.