High-efficiency phosphorus removal is generally found in the sequencing batch biofilm reactor ( SBBR) without sludge discharge for the treatment of high-concentration domestic sewage,which can not be explained by the traditional PAOs ( Poly-Phosphate Accumulating Organisms) phosphorus removal theory. In order to explore the way to the high-efficiency phosphorus removal without sludge discharge,the phosphorus balance of the system was analyzed,and the contents of ploy-P and matrix-bound phosphine ( MBP) were monitored. Experimental results indicate that the effluent total phosphorus ( TP) removal stabilizes at 82. 1%~91. 8% in the condition of continuous aeration during a 38-d continuous operation without sludge discharge. According to the analysis results of phosphorus balance in the period from the system start-up to a 45-d running,it is found that 530. 0mg of phosphorus is lost in the system,with only 0. 257 mg of poly-P contained in per gram of dry sludge,and the TP content reaches 1. 93% ( calculated as the mass fraction of TP in per gram of dry sludge) ,which means that the discussed system is a non-PAOs biological phosphorus removal system. Moreover,it is found that the phosphorus in the system can be transformed into phosphine gas as the MBP mass in per kilogram of wet sludge reaches up to 3. 5mg.