The invention discloses a liquid-phase mineralizing precursor and a method of repairing demineralized dentine and belongs to the technical field of biomaterials. The liquid-phase mineralizing precursor can repair the demineralized dentine and is a calcium carbonate mineralizing precursor solution containing a polymer and magnesium ions, wherein the concentration of the polymer is 5-100 [mu]g/mL, concentration of Ca<2+> is 1-15 mM, and molar ratio of Mg<2+> to Ca<2+> is 1-6:1. A preparation method of the liquid-phase mineralizing precursor includes steps of: suspending CaCO3 powder in distilledwater and feeding CO2 gas for 1-4 h at room temperature; filtering away residual CaCO3 and feeding CO2 gas for 15-60 min at room temperature; measuring the concentration of Ca<2+> and adding distilled water to regulate the concentration of Ca<2+>; adding the polymer and MgCl2 powder and uniformly mixing the mixture. The liquid-phase mineralizing precursor employs easy-to-obtained raw material andhas simple system, and is a creative break for therapy on oversensitive dentine.