To optimize the flow field of selective catalytic reduction (SCR) DeNO_x reactor, a combinational optimizing scheme was adopted. A multi-orifice plate was arranged at the inlet of the reactor, and three kinds of deflector were arranged inside the reactor: gradual expansion type, grid type and arc type. The flow field inside the reactor was obtained by numerical simulation using computational fluid dynamics (CFD), and then the comparative study was carried out. The results showed that the pressure drop was small and the performance of distributing flow field was excellent, when the multi-orifice′s ratio value of hole pitch to diameter was 1.2. The deflectors designed based on the structure of reactor had better performance. The catalyst had the ability of distributing flow field. Arranging catalyst in multiple layers could effectively optimize the flow fields of the beds after the first layer, but increase the pressure drop at the same time. The relative standard deviation (RSD) of flow velocity of the first layer catalyst bed decreased from 7.16% to 2.51% by adopting the combined diversion scheme, and the dissipation of turbulent kinetic energy was reduced. The research results have a guidance for the design of the SCR reactor′s deflectors.