The invention discloses a method for mineralizing CO2 with blast iron slag and co-producing alumina. The method comprises the following steps: (1) evenly mixing blast furnace slag with ammonium sulfate; (2) roasting mixed materials in a muffle furnace to obtain roasted slag and ammonia water obtained by absorbing tail gas during absorption with water; (3) performing water logging to the roasted slag, and filtering to obtain water logging slag containing CaSO4 and SiO2 and water extract rich in Mg and Al; (4) adding ammonium water into liquid rich in Mg and Al, controlling the pH value to obtain parent liquor containing Al(OH)3, Mg(OH)2 and ammonium sulfate; (5) blending the water logging slag into pulp with ammonium water, introducing CO2 for mineralizing reaction, and filtering to obtain mineralized slag containing CaCO3 and SiO2 and mineralized parent liquor containing ammonium sulfate; (6) mixing Mg(OH)2 into slurry with water, mineralizing to obtain MgCO3; (7) calcining Al(OH)3 to obtain byproduct Al2O3; and (8) mixing and evaporating the parent liquor, cooling, crystalizing and recovering ammonium sulfate. According to the method, the conditions are mild, the process cost is low, not only can large-scale CO2 emission reduction be realized, but also valuable element in blast furnace slag can be recovered, and the high-value use of the blast furnace slag can be realized.