The experiment was conducted to determine effects of protein levels and satiation degrees on juvenile Cynoglossus semilaevis Günther in industrial culture conditions with closed recirculation seawater.The 5×3 two-factor random animal experiment lasted for 108 days,total of 1125 fish［with initial weight(110±25) g］were randomly allotted into 15 treatments with triplicates,25 fish each replicate,5 protein levels diets(43%,46%,49%,52%,56%,with A-E),3 satiation degrees feeding(100%,90%,80%,withⅠ,Ⅱ and Ⅲ).Evaluated effects of different protein levels and satiation degrees on juvenile C.semilaevis growth performance,seawater environment factors and digestive enzyme.The primary results showed that:(1)As the protein level of feed and satiation degree increased,weight gain increased significantly(P<0.01),the best weight gain both in the highest protein level and the highest satiation degree.Weight gain ratio in group E was significantly higher than the other groups by 13.75%-50.16%,that in degree Ⅰ was 7.57% and 14.08% higher than degree Ⅱand Ⅲ .The best feed conversion ratio appeared in the highest protein level,which was significantly higher than the other groups by 6.25%-27.44%(P<0.05),but satiation degree didn’t have such significant impact on FCR as protein level did:there was no significant difference between degreeⅠ and Ⅱ,and Ⅰ did better on FCR than Ⅲ significantly(P<0.05).As the protein level and satiation degree increased,NH⁺₄-N,NO^-₂-N excretion and total baneful nitrogen excretion were enhanced(P<0.01).The total baneful nitrogen excretion in group C was 54.5% and 64.4% lower than group D and E respectively,that in degree Ⅰ was 17.8% and 29.2% higher than Ⅱand Ⅲ ,respectively;(2)The activity of protease enhanced,lipase reduced and amylase increased at first,then reduced with increased protein level.As satiation degree increased,digestive enzyme reduced.digestive enzyme of liver was higher than intestine;(3)The relationship between nitrogen excretion and the weight gain as the increasing dietary protein levels,partially accorded with the Rhomb character.By regression analysis,the optimum econutrition requirement of protein for juvenile C.semilaevis seemed to be 48.30%,49.27% and 50.67%,respectively in degree Ⅰ,Ⅱ and Ⅲ.The optimum eco-aquatic requirement of satiation was 90%.