Abstract: The effects of stable operated biological filter under four NH₄-N concentrations on purification of mariculture external drainage were analysed on the conditions of seven different aeration ratios.The results showed that:during the start-up stage of biological filter, the biofilm mature period shortened with increasing NH₄-N concentrations.The biofilm mature period of filter under higher NH₄-N concentration was ten days shorter than the other three filters of low NH₄-N concentrations.After the biological filter was started successfully, with the gradual increase in the NH₄-N concentrations, the capacity of filters under the same aeration ratio in removing effluent NH₄-N decreased from 81.1% to 73.3%.Compared with the other three filters of low NH₄-N concentrations, the effluent NO₂-N concentrations of the filter under higher NH₄-N concentration increased by ten times and eventually reached 2.6 mg/L.In the four biological filters, the mean removal rates of effluent NH₄-N first increased and then decreased with the increasing aeration ratios.The average removal rates of effluent NH₄-N achieved the highest when the aeration ratio was 10:1.Therefore, the ideal aeration ratio for the operation of biological filter was 10:1 and the suitable NH₄-N concentration was selected as 4.0 mg/L.Under these conditions, the biological filter has a shorter start-up period, which reached the best removal efficiency of effluent NH₄-N and lower concentration of effluent NO₂-N, so as to achieve the best purification effect on the mariculture external drainage.