The treatment of middle-stage effluent in paper mills with inter loopaerobic three-phrase fluidized bedis carried out.After aperiod of domestication,the character sofdis charged water are stable.The COD removal rate keeps higher than65%,which in dicates that the system could sustain the variation of input load.The increase of[Cl^-]ineffluentimpliedthe biofilm could biodegrad echloroa romatic component sincert a indegree after domestication.The best ratioof liquid-gas should be controlled betweent/120and1/140.Theout flow off luidizedbed is treated by flocculation,and the final drains COD is60~80mg/L,BODis50~60mg/L,chroma is 100~150CU,volatility-hydro xybenzene is less than 0.0265mg/L.