介绍了采用冷凝控制法和80%异丙醇吸收法分别对选择性催化还原（selective catalytic reduction，SCR）脱硝系统烟气中的SO3进行采样，再用高氯酸钡－钍试剂法分别测定烟气中SO3浓度的过程。对2种采样方法的准确度以及影响因素进行了试验研究。实例试验结果表明，使用冷凝控制法采样测得的SO3浓度偏低，使用80%异丙醇吸收法采样，再用高氯酸钡－钍试剂测定SO3浓度的方法简单、快捷、准确，便于现场监测控制。

The sampling and concentration determination of SO3 in flue gas for selective catalytic reduction (SCR) denitration system is introduced, separately with controlled condensation method and 80% isopropanol absorption method to sample and barium perchlorate-thorin indicator titration method to measure. The accuracy and influencial factors of these two sampling methods were studied and analyzed. The experimental results show that the concentration of SO3 sampled with controlled condensation method is lower than that with 80% isopropanol absorption method. The method that firstly samples SO3 with 80% isopropanol absorption method, then measures the SO3 concentration with barium perchlorate-thorin method, is simple, fast, accurate, and easy to on-site monitoring.