The invention relates to a preliminary dexidation-anaerobism-oxygen deficit-multistage aerobic / anaerobic biological N and P removing technology, including the steps: a) deoxygenation denitrification is carried out on returned sludge and part of sewage; b) anaerobic phosphorus releasing is carried out on the mixed liquid and part of sewage treated by the step a; c) denitrification is carried out on the mixed liquid and part of sewage treated by the step b and returned nitrification liquid; d) synchronous nitrification denitrification and decarbonising phosphorous absorption is carried out on the mixed liquid treated by the step c; e) nitrification liquid treated by the step d is returned to the step c; f) nitrification liquid treated by the step d is precipitated, part of sludge is returned to the step a, excess sludge is discharged, clean water is discharge or recycled after reaching standards. The device comprises a preliminary dexidation tank, an anearobism tank, an anaerobic tank, a multistage aerobic / anaerobic tank and a secondary sedimentation tank which are sequentially communicated; wherein the multistage aerobic / anaerobic tank is internally provided with a suspension chain aeration system, an inflow water distributing pipeline is connected with the preliminary dexidation tank, the anaerobism tank and the anaerobic tank, and sludge reflux quantity and nitrification liquid reflux quantity can all be regulated by a refluxing device.