The invention discloses a system and a method for deeply treating tail water of a sewage plant. The system comprises a high-density settlement tank, a powder activated carbon membrane reactor, a resinadsorption tank, a clear water tank, a waste liquid collection tank, a denitrification tank, an MBR tank, a sludge concentration tank, a plate and frame filter press, and an electric control system.Raw water enters the high-density settlement tank at first so as to remove total phosphorus, then the water is sent to the powder activated carbon membrane reactor, powder activated carbon is added toadsorb pollutants, the water is filtered through a membrane component so as to reduce CODcr, color scale and suspended matters of effluent water, then the effluent water is sent to the resin adsorption tank to remove nitrogen, the effluent water flows into the clear water tank, is sterilized and then is discharged, after being saturated through absorption, the resin is regenerated through a sodium chloride solution, the waste liquid is collected and then sent to the denitrification tank to remove nitrogen, some carbon paste in the powder activated carbon membrane reactor in the previous stageflows back to the denitrification tank, so as to enable pollutants to be further adsorbed to improve water quality, then the effluent water enters the MBR to separate sludge from water. Through adoption of the system, the water quality is stable, the effluent water contains low content of nitrogen and phosphorus; and the system occupies a small area and has high automation degree.