Due to the complex situation of flow field in up-flow staged sludge bed,the paper applied Euler multi-phase model to investigate the effect of factors such as inflow modes, flow baffles structure and water inlet velocity on 3D numerical flow field in USSB reactor through numerical simulation.The result showed that the reactor presents the better complicated flow state under the structure form of partial inflow and wide backflow seam about 50mm;when the inlet velocity is 0.001m/s, the sludge bed layer not only keeps the better floating state, but also forms the suspended region of granular sludge in the upper area of reactor, which can achieve full contact and mixing between wastewater and granular sludge in the reactor.