Micro/nano magnesium carbonate pentahydrate (MgCO₃·5H₂O) with flower-like morphology was synthesized using magnesite as a substrate and potassium dihydrogen phosphate as an additive. The synthesized samples were characterized by scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetry and differential scanning calorimetry. The influence of pyrolysis time on crystal morphology was explored. The formation mechanism was investigated on the basis of the characterized results and the crystal structure of MgCO₃·5H₂O. The results showed that the flower-like MgCO₃·5H₂O was 1.5–3.0 μm in length and 100–500 nm in diameter and was successfully obtained with a pyrolysis time of 30 min. The formation mechanism of flower-like MgCO₃·5H₂O is suggested to be the selective adsorption of potassium dihydrogen phosphate on the surface. The process of flower-like crystal growth is as follows: amorphous nanoparticles formation, acicular and rod monocrystal formation, flower-like monocrystal formation, and flower-like polymers (MgCO₃·5H₂O) crystallization. In the MgCO₃·5H₂O crystal, the magnesium ion presents two different octahedral coordinations corresponding to Mg(H₂O)₆²⁺ and [Mg(H₂O) (CO₃²⁻)₂]²⁻, and the chemical formula of the crystal is Mg(H₂O)₆ · Mg(H₂O)₄(CO₃²⁻)₂.