Selective catalytic reduction (SCR) and denitrification are the best technologies for nitrogen oxides (NO_x) control in coal-fired power plants, and their denitration efficiency and ammonia escape rate are closely related to their internal flow characteristics. By adding a deflector to the SCR device, the flow field in the curve can be effectively improved, and the stable and efficient operation of the SCR device can be realized. Based on the numerical simulation method, the SCR system of a coking coal-fired boiler in a steel plant was simulated using k-ε (the turbulence model), and three design schemes of deflectors were proposed and numerically simulated simultaneously. After optimization, the ammonia injection grid’s downstream velocity variance coefficient C_V was 6.69, the catalyst upper cross-section velocity variance coefficient was 11.84, the cross-sectional temperature average was 499 K, the maximum temperature deviation was 9 °C, the maximum-to-minimum temperature interval span was 15 °C, the cross-sectional NH₃/NOx molar ratio average value was 0.8122, the coefficient of variance was 4.67, and the pressure loss was 1855 Pa. The findings of this work will help improve the denitration efficiency and provide an important reference for the actual transformation design.