A novel AF-MBMBR system (sponge moving bed membrane bioreactor coupled with a pre-positioned anoxic biofilter) was proposed for treating saline wastewater from mariculture. Its development process was divided into 4 stages and the performance in each stage were evaluated in order to clarify the pollutants removal mechanism and the membrane filtration performance. The results showed that TOC removal efficiency was maintained at high level (92.8%-96.2%) throughout the development period due to the utilization of pre-acclimated biomass; Excellent TN (total nitrogen) removal efficiency (up to 93.2%) was progressively obtained which could be attributed to the combined effects of pre-positioned AF and sponge bio-carriers in MBMBR. Indeed, such an effective TN removal for the treatment of saline wastewater was rarely reported. Moreover, the short-cut nitrification-denitrification was observed to be the dominant nitrogen removal pathway in AF-MBMBR. Concerning membrane filtration performance, the filtration cycle was pronouncedly prolonged from 10 days to 44 days. Such an improvement was the result of mitigation on cake layer resistance, which could be attributed to the reduction of MLSS and organic load in MBR induced by pre-positioned AF, as well as the modification effect of sponge bio-carriers on mixed liquor in MBR.