To determine the dietary protein requirement of mandarin fish （Siniperca chuatsi） with different sizes，mandarin fish with initial body weight of （11.58±1.34） g （G1），（94.77±2.59） g （G2） and （245.26±3.59） g （G3） were selected to investigate the effects of different dietary protein levels on growth performance，feed utilization rate，morphological indexes，digestive enzyme activity，antioxidant capacity and ammonia nitrogen excretion.The results showed that when the protein content was 500 g/kg，the specific growth rate （SGR），final body weight （FBW） and weight gain rate （WGR） of mandarin fish in G1 and G2 groups were the highest，while when the protein content was 450 g/kg，those in group G3 were the highest.The feed coefficient showed an opposite trend.Based on the analysis of SGR’s broken-line regression model，the optimal protein requirements of three different sizes of mandarin fish were 497.1 g/kg，451.9 g/kg and 446 g/kg，respectively.The survival rate of mandarin fish with small and middle sizes fed with low protein level （350 g/kg and 400 g/kg） diet was significantly lower than that of other groups （P<0.05），but dietary protein level had no significant effect on survival rate of mandarin fish in G3 group （P>0.05）.Condition factor，hepotasomatic index and viscerasomatic index of mandarin fish in G1 and G2 groups decreased significantly with the increase of dietary protein level （P<0.05）.With the increase of dietary protein level，the dry matter apparent digestibility coefficient （ADC） of mandarin fish was first increased and then stabilized in G1 and G3 groups，while that in G2 group was first increased and then decreased.With the increase of dietary protein level，the crude protein ADC of mandarin fish with three different sizes increased significantly （P<0.05） and decreased slightly after reaching the highest value at 500 g/kg protein level.A similar trend was observed in trypsin activity of the foregut.The activities of glutamic-pyruvic transaminase and glutamic-oxalacetic transaminase in liver of mandarin fish at three different growth stages increased dramatically as dietary protein level elevated （P<0.05）.The ammonia nitrogen excretion rate of mandarin fish with three different sizes reached the peak at 6 h after feeding and increased significantly with the increase of dietary protein level （P<0.05）.When feeding with high protein level diet，the ammonia nitrogen excretion rate of mandarin fish in G3 group was noticeably higher than that in G1 group （P<0.05）.The above results showed that dietary protein content had significant effects on the growth performance，digestion and metabolic indexes of mandarin with three sizes，and the optimal protein requirement could be determined to achieve the optimal healthy breeding effect for mandarin fish.