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.