CO2 mineralization is conducted through injecting simulated flue gas into MgSO4‐(NH4)2SO4‐NH3·H2O. The MgCO3·3H2O whiskers are prepared by adjusting reaction conditions, including mineralization pH, reaction temperature, and amount of ammonia. The chemical component and morphology of the product are analyzed via scanning electron microscopy, X‐ray diffraction, and thermal gravimetric analyzer. The study demonstrates that at 50°C, pH 9, 0.08 L/min CO2 flow rate, and a simulated flue gas containing 20% CO2, MgCO3·3H2O whiskers of 99% purity are synthesized. The whiskers have a length of 15–20 μm with an aspect ratio of 18. At ammonia usage of 25 ml, the Mg2+ conversion rate is 70%.