Multiple-time addition of carbon sources, and the shortages and overages of it can be avoided for a denitrification progress using biological degradable Polymers(BDPs) as organic carbon sources. It has been proven that Polycaprolactone(PCL) can act as organic carbon sources for the denitrification progress in the recirculating aquaculture freshwater. In this paper, the effect of additive amount of PCL on the nitrate-nitrogen removal in the recirculating aquaculture freshwater was studied. In a denitrification progress with a about 0.1g/L load of nitrate nitrogen in the influent water, when the additive amount of PCL in a water body of 200ml increased from 5 g, 10 g, 15 g, 20 g, 25 g and 30 g, the removal efficiency of nitrate-nitrogen for each group showed no significant difference, but the concentration of the unused dissolved organic carbon (DOC) increased with the increasing of the additive amount of PCL. It showed that the change of additive amount of PCL did not affect the removal efficiency of nitrate-nitrogen, but had a significant effect on the accumulation of unused dissolved organic carbon (DOC). In conclusion, 5 g is the best suitable additive amount for PCL.