Abstract: In order to improve the flow field distribution in a selective catalytic reduction（SCR） flue gas deNOx system flow channel, the flow of the flow channel was numerically simulated by ANSYS-Fluent software, 18 sets of calculation conditions were designed by orthogonal test method, and the influence of velocity uniformity through deflectors with different structural parameters, porous plates, rectifier grids to the ammonia injection grid（AIG） upstream and the entrance of the first layer catalyst were analyzed. The simulation results show that: affecting the velocity uniformity of AIG upstream greatly is the open ratio of porous plate, affecting the velocity uniformity of entrance of the first layer catalyst greatly is rectifier grid spacing; under the optimization scheme, the relative standard deviation coefficient Cv1 value of the AIG upstream is 3.94%, the relative standard deviation coefficient Cv2 value of the entrance of the first layer catalyst is 4.33%, the system pressure drop is 517 Pa.