After the boiler of a thermal power plant in Nanjing was reformed for denitrification, the clogging of fly ash particles occurred near the support beam of the economizer. The critical speed criterion under different working conditions was constructed by Fluent custom code (UDF), and the change of fly ash deposition on the support beam of the economizer was simulated without ABS and with ABS. At the same time, the influence of the fin layout structure on the movement of smoke and fly ash particles was analyzed. The results show that the stagnation of fly ash particles on the supporting beam is the main cause of sediment clogging. Due to the production of ABS in the process of denitrification, the adhesion of fly ash particles is intensified. At the same time, the fin structure on the support beam hinders the lateral movement of fly ash particles, which causes the growth of clogged fly ash near the support plate. On this basis, an optimization plan for the fin structure is proposed, which improves the flue gas flow conditions and avoids the occurrence of clogging and growth of fly ash.