This study presents how the UV device affecting the Cynoglossus semilaevis’s RAS environment and its growth by arranging the device in the front end and rear end of biofilter.The results showed that ：a. the change of positions of UV device has no significant effects on sterilize the bacteria and Vibrio，but it has huge impact on the number of bacteria and Vibrio in culture pond；b. when the UV device was placed on the rear end，the biofilter’s removal rate of NH₄⁺ -N，NO₂^- -N，COD was 35.81 %，6.67 %，12.00 %，lower than when it was set on the front end，which is 51.51 %，11.54 % 42.45 %；c. when the device was in the front end，the mean concentration of ammonia nitrogen and nitrite nitrogen was down by 9.7 %，21.6 % than it was on the rear end；d. the change of positions of UV device affects the growth rate of the fish remarkably，The special growth rate (SGR) for the two group respectively were 0.99，0.21，The lead of fish weight gain rate was 4.27 % significantly below the rear of 21.71 %；e.the difference between the two groups was obvious；the number of erythrocyte，leucocyte and hemoglobin of the fish were much lower than the rear group.