The invention discloses a front oxygen-lack A2O denitrifying and dephosphorizing device coupled with an S-UASB (Upflow Anaerobic Sludge Blanket) sludge anaerobic fermentation device. The front oxygen-lack A2O denitrifying and dephosphorizing device comprises an SBR (Sequencing Batch Reactor) sludge pretreatment device, a UASB anaerobic treatment device, a water collecting pool, a front oxygen-lackA2O device and a secondary precipitation tank, wherein the SBR sludge pretreatment device comprises an SBR; the UASB anaerobic treatment device comprises an UASB anaerobic reactor; an SBR sludge outlet is connected into a UASB sludge inlet; the SBR is also connected with an agent box; a UASB water outlet is connected into the water collecting pool; the front oxygen-lack A2O device comprises a reaction tank body which is partitioned into a first oxygen-lack area, an anaerobic area, a second oxygen-lack area and an aerobic area; a water outlet connected into the secondary precipitation tank isformed in the upper side wall of the aerobic area. Meanwhile, the invention provides a method for treating sludge. The front oxygen-lack A2O denitrifying and dephosphorizing device and the method fortreating the sludge disclosed by the invention have the beneficial effects that not only is the problem of lower denitrifying and dephosphorizing efficiency caused by lack of a carbon source in the front oxygen-lack A2O device solved, but also the residual sludge produced is effectively treated, the sludge treating and disposing cost is reduced, and the pollution to the environment is reduced.