SO3 exists in the atmosphere in the form of sulfuric acid aerosol, heavily polluting the environment and decreasing the safety of boiler equipment. This study focuses on the homogeneous formation mechanism of SO3 in the post-flame region. We conducted experiments and simulations to investigate the influence of various combustion parameters on SO3 generation. The formation of SO3 was affected by factors such as temperature and the concentrations of O2, SO2, NO, and H2O. With a rise in temperature, the SO3 concentration initially increased but then decreased, reaching its maximum at about 1000°C, which indicates that SO2 can promote the formation of SO3 over a certain temperature range. An increase in the O2 concentration promoted the formation of O and OH radicals, which enhanced the generation of SO3 from SO2. The presence of NO resulted in direct and indirect interactions between NOx and SOx species for different reaction sets, potentially enhancing SO3 generation. With an increase in the H2O concentration, SO3 formation initially increased rapidly before plateauing. ROP (rate of production) and sensitivity analyses suggest that adding H2O will produce O and OH, which strongly influence SO3 formation. Furthermore, the sensitivity analysis indicated that radicals and the direct reaction between SO2 and NO2 play significant roles in SO3 formation.