In order to accurately predict the acid dew point of coal-fired power plant boiler flue gas and seek a more suitable formula for calculating the acid dew point of flue gas in actual production, the least square method is used to compare the common acid dew point calculation formulas based on relevant test data. And obtain the acid dew point calculation formula based on sulfur trioxide. The fitting formulas of Taylor and Xiang Boxiang’s data tables were obtained by fitting and compared with common calculation formulas. The results show that the calculation form of Verhoff&Branchero calculation formula is more suitable for the simulation of acid dew point calculation formula based on sulfur trioxide. Together, Taylor’s calculation formula has a wider range of values and is more suitable for field applications, But when the sulfur trioxide concentration is lower than 20ppm, the Haase calculation formula has higher calculation accuracy.