The invention relates to a dry flue gas desulphurization method using steel slag or blast furnace slag as absorbing agents, which comprises the following steps: adopting a nitrification circulation fluidized bed flue gas desulphurization device; selecting the steel slag or the blast furnace slag with the granularity smaller than or equal to 0.048 mm to be sprayed into a desulphurization tower by a pneumatic conveying method; spraying process water into the desulphurization tower through an atomization water spraying nozzle; introducing the flue gas into the desulphurization tower from a flue gas inlet of a pre-dust-removal section; making the desulphurized flue gas enter a secondary dust remover; spraying circulation dust into the desulphurization tower by the pneumatic conveying method; discharging the dust into an external transportation dust field from an external dust discharge outlet; and discharging the clean flue gas into a chimney through a draught fan to be discharged into the atmosphere. The invention adopts the steel slag or the blast furnace slag as the absorbing agents, the desulphurization cost is low, the nitrification circulation fluidized bed flue gas desulphurization device is adopted, ground occupied by the system can be saved, the equipment investment can be reduced, and in addition, the desulphurization effect is good.