The present invention relates to a device for efficiently preparing magnesium hydroxide, comprising a material mixing tank, a coordinated pressure control unit I, a reactor, a coordinated pressure control unit II, a condenser, a three-way valve, a material storage tank, a coordinated pressure control unit III and a steam generator. A outlet of the material mixing tank is connected with one end of the coordinated pressure control unit I by a pump; the other end of the coordinated pressure control unit I is connected with an inlet of the reactor; the outlet of the reactor is connected with one end of the coordinated pressure control unit II; the other end of the coordinated pressure control unit II is connected with one end of the condenser; the other end of the condenser is connected with a first port of the three-way valve; a second port of the three-way valve is connected with a material inlet of the material mixing tank; a third port of the three-way valve is connected with the material storage tank; a steam inlet of the reactor is connected with one end of the coordinated pressure control unit III; and the other end of the coordinated pressure control unit III is connected with the steam generator.