The invention discloses a CO2 hardened inorganic binder and a preparation method thereof. The liquid binder comprises a sodium silicate solution, a potassium silicate solution, aluminum phosphate, potassium hydroxide and sodium hydroxide. A powder accelerator comprises silica fume, calcium oxide, titanium oxide, zirconium oxide, zinc oxide, yttrium oxide and graphite. The preparation method comprises the following steps: adding a sodium silicate solution, a potassium silicate solution and sodium hydroxide into a container, stirring, heating the liquid in the container, adding aluminum phosphate, continuously heating, adding potassium hydroxide, continuously stirring, cooling the liquid to the room temperature to obtain the liquid binder; and uniformly stirring silica fume, calcium oxide, titanium oxide, zirconium oxide, calcium oxide, yttrium oxide and graphite to obtain the powder accelerator. The inorganic binder is low in cost and high in environment friendliness, the organic binderenables the molding sand to be high in surface quality and good in surface stability, organic components are added, the residual strength of the molding sand is greatly reduced, the casting production of high manganese steel casting and product quality are improved, and the inorganic binder is applied to the technical field of casting.