Abstract: Combined biofilters consisting of an ulva biofilter(UB), a polishing ulva biofilter(PUB) and a sand biofilter(SB) were developed for purification of the rearing water and effluent water in recirculating abalone culture system. UB exhibited intensive uptake to total ammonia-nitrogen(TAN) in the rearing water, thereby reduced the nitrification loadings of SB and lowered accumulation of TAN, NO^－₂-N, NO^－₃-N and COD in the system. During the experiment, the concentration of TAN, NO^-₂-N, NO^-₃-N and COD in the rearing water was kept below 0.19, 0.01, 1.75 and 1.20 mg/L, respectively. Due to the uptake of UB to PO^3－₄ and the nitrification of SB, the concentration of PO^3－₄ in the rearing water retained to 0.30 mg/L during the operation period. The combined biofilters also had good capability of pH value buffering in the rearing water and were able to maintain the pH value between 8.11～8.14 during the whole experiment. The concentration of PO^3-₄ in the effluent water dropt to 0.22 mg/L and NO^-₃-N level even dropt to 0.10 mg/L after further uptake by PUB. The inorganic nitrogen cycling models were established for estimating the changes of TAN, NO^-₂-N and NO^-₃-N in the rearing water.