Abstract: MgCO₃·3H₂O crystals were prepared by thermal decomposition method with the magnesium solution as precursor, calcium chloride as additives. The influence of types of additives, the amount of calcium chloride and pyrolytic time on the crystallization process was investigated. The mechanism of calcium chloride was further analyzed. The phase structure and morphology of the products was characterized with X-ray diffraction (XRD) and scanning electron microscope (SEM). The results show that rod-like MgCO₃·3H₂O crystals with spherical particles on the surface was obtained when the amount of calcium chloride is 5.0 g/L. The average length and aspect ratio of rod-like crystals is 45 μm and 15. The calcium chloride has no regulation to the directional growth of MgCO₃·3H₂O crystals. The calcium chloride takes part in the crystallization of MgCO₃·3H₂O crystals with the electrostatic adsorption. The induction period is prolonged, the nucleation rate decreases and the growth rate increases with the participation of calcium chloride.