The fractional crystallization of carnallite (KMgCl3·6H2O) in pure water, KCl aqueous solution and MgCl2 aqueous solution was studied. When carnallite was dissolved in pure water, porous crystals of KCl were formed. However, when the fractional crystallization in KCl or MgCl2 aqueous solution occured, the solid crystals of KCl with different morphologies were formed. A theoretical model was established to explore this phenomenon. The ionic product of carnallite and KCl with distance far from the surface of carnallite was analyzed in which the activity coefficient of KCl, MgCl2 and water were calculated with Lu-Maurer model. The dissolution of carnallite, the diffusion of KCl and MgCl2 in bulk solutions and the nucleation as well as crystal growth of KCl were investigated in detail. It was concluded that the coupling of the high dissolution rate of carnallite and the high nucleation rate of KCl nearby the surface of carnallite resulted in the porous crystals of KCl. TG and SEM analysis shows that high water content in KCl crystals prepared by fractional crystallization of carnallite in water results from the surface porous structure of crystals.