The SO₃ in coal-fired flue gas is easy to condense, reduce the smoke emission transparency and cause corrosion on the surface of the equipment. With the urgency of accurate measurement and effective treatment of SO₃, this article focuses on the development of the controlled condensation method (CCM) for SO₃ testing. An SO₃ test accuracy verification device is set up for evaluating the test accuracy of CCM, and the influence of factors on the accuracy of CCM is performed, such as sampling a gun’s heating temperature, sampling flow rate, high SO₂ concentration and difference of SO₃ concentration during the test procedure. The CCM can effectively avoid the interference of SO₂ on capture efficiency. With the increase of heating temperature and flow rate, the capture efficiency first increases and then decreases. Reasonable control of the tracing temperature and sampling flow rate helps to improve the accuracy of CCM. In a certain range of SO₃ concentration, the capture efficiency has a linear relationship with SO₃ concentration. The finding of the current study shows that this is feasible and provides significant references as to how to improve SO₃ detection accuracy.