The present invention relates to a method for preparing a transparent magnesium hydroxide liquid phase dispersion. The method comprises: 1) dissolving a magnesium salt in water or an organic solvent to obtain a magnesium salt solution, and dissolving an alkali in water or an organic solvent to obtain an alkali solution; 2) adding the magnesium salt solution and the alkali solution to a supergravity rotation filling bed or a casing type annular micro-channel reactor to carry out a reaction to obtain a magnesium hydroxide suspension; 3) adding a surfactant to the magnesium hydroxide suspension to modify, and standing the obtained modified solution; and 4) filtering and washing the modified solution, and dispersing in a liquid phase medium to obtain the transparent magnesium hydroxide liquid phase dispersion. According to the present invention, the method is performed in the supergravity rotation filling bed or the casing type annular micro-channel reactor so as to substantially enhance mass transfer and enhance micro-mixing; the product is transparent and has good dispersity; the magnesium hydroxide crystal has characteristics of small particle side, uniform distribution, and one-dimensional size of 0.5-30 nm; and the prepared product has the solid content of 1-50 wt% and good stability, and does not produce settlement after standing for over 10 months.