Abstract: Ammonia co-firing in coal-fired boilers significantly alters the flue gas composition and heat transfer characteristics of the boilers, but its effect on the radiative properties of flue gas remains unclear. Based on the Leckner radiation model, the radiative properties of coal-fired flue gas with high water vapor concentration were calculated, and the influence of ammonia co-firing at 0-60% thermal ratio on heat transfer of the heating surfaces in a 600 MW subcritical boiler under 50%-100% load was investigated. Results show that at 20%-60% ammonia co-firing ratio, the concentrations of fly ash and char particles in the flue gas decrease substantially by 15.9%-72%, while the adiabatic combustion temperature drops by 47-133 K, resulting in a 3.3%-9.8% reduction in furnace radiative heat transfer power. After ammonia co-firing, the flue gas flow rate increases by 4%-12%, enhancing convective heat transfer and causing an overtemperature trend in the primary and reheat steam. By lowering the burner tilt angle by 6°-19°, the primary and reheat steam temperatures can be restored to the design values. At 60% ammonia co-firing ratio, the exhausted gas temperature increases by 11.6-13.1 K, and the boiler efficiency decreases by 0.9%-1.1%. The findings provide a theoretical basis for large-ratio ammonia co-firing operation and heat transfer regulation in coal-fired boilers.