Abstract: Taking the selective catalytic reduction (SCR) reaction system of a 300 MW coal-fired boiler as an example, numerical simulation was made to the denitration characteristics of the SCR reactor with different structures by fluid mechanics calculation software, so as to analyze the effects of guide plate layout on distribution of both the flow field and the ammonia-nitrogen ratio in the SCR reactor. Results show that both the flow field and NH3 volume fraction within the SCR reactor are severely uneven, the denitration efficiency of SCR reactor is relatively low and the ammonia escape rate is relatively high, when there is no guide plate installed in the flue; by adopting the initial solution, the flow filed in the SCR reactor is improved greatly, but the deviation of ammonia-nitrogen ratio distribution at inlet of the first catalyst layer is more than 5%; whereas, in the final solution, the deviation of velocity distribution at inlet of the first catalyst layer is 3.84%, where the deviation of ammonia-nitrogen ratio distribution is 3.79%, the denitration efficiency of SCR reactor reaches 83%, and the NH3 escape rate is lower than 5×10-6