The invention discloses a preparation method of monoclinic-phase nano zirconium dioxide, and the monoclinic-phase nano zirconium dioxide prepared therethrough. The preparation method includes the following steps: 1) contacting a solid mixture containing a zirconium source and a solid organic amine with water vapor in a sealed container to prepare zirconium hydroxide, the temperature of the water vapor being 100-250 DEG C; and 2) drying and roasting the zirconium hydroxide to prepare the monoclinic-phase nano zirconium dioxide. The method improves the pore structure of the zirconium dioxide and increases yield of the zirconium dioxide, is free of product separation and reduces wastewater treatment.