Based on the thermal calculation, the paper makes a contrastive analysis on the parameters of flue gas, and convection heat properties of the coal-fired boiler under atmospheric air combustion, atmospheric oxy-fuel combustion and pressurized (6MPa) oxy-fuel combustion conditions. It takes a 300MW pressurized(6MPa) oxy-fuel combustion boiler as research object, the result indicates that: compared to the coal-fired boiler atmospheric air combustion, the flue gas volume flow in the pressurized oxy-fuel combustion has a decrease of 98.79%; convective heat output has a decrease of 24.69% with the same difference in temperature. In the pressurized oxy-fuel combustion, both the flue gas convective heat transfer coefficient and the pressure drop are greater than the atmospheric oxy-fuel combustion, flue cross-sectional area is smaller than conventional boiler, and heating surface area is less than atmospheric oxy-fuel combustion. With a method named dynamic minimization of costs the best flue gas velocity in this paper is 1.07m/s
