Abstract: A selective catalytic reduction (SCR) system, comprising a box body, a mixer, a baffle and a carrier, the mixer, the baffle and the carrier being sequentially arranged inside the box body in an airflow direction inside the box body, the baffle being provided with a plurality of through holes, and the distribution density of the through holes and/or the hole area of the through holes being negatively correlated with a distribution uniformity index of NH3 at the baffle. The configuration of the distribution density and the hole area of the through holes can improve the mixing uniformity of ammonia gas, so that a reaction effect of a catalyst and a mixed gas in the carrier is enhanced. The settings of the distribution density and the hole area of the through holes can improve the mixing uniformity of an ammonia gas.