The invention relates to preparation methods of rare earth layered hydroxide nanosheets and rare earth layered hydroxide nanosheet sol. The preparation method of the nanosheets comprises the followingsteps: preparing a lanthanum nitrate solution with a corresponding concentration, dissolving ammonium sulfate particles into the lanthanum nitrate solution according to a proportion so as to form a mixed solution, adjusting the pH of the mixed solution to 6-10, and continuously stirring to obtain uniform suspension; carrying out a reaction to obtain a product, immediately carrying out centrifugalseparation on the product, and drying to obtain the rare earth layered hydroxide La2(OH)4SO4.nH2O nanosheets. The rare earth layered hydroxide nanosheet sol is prepared by putting the prepared nanosheets into a formamide solution, carrying out ultrasonic centrifugal treatment, and then taking the supernatant. According to the preparation method, the rare earth layered hydroxide La2(OH)4SO4.nH2O nanosheets can be prepared in one step by using an ice-bath method, so that the problem that in the prior art, the compounds cannot be prepared into single-layer nanosheets by using the conventional intercalation-stripping method can be solved; furthermore, the preparation methods are simple in process and convenient to operate, and can realize mass production; the corresponding nanosheet sol is good in stability and material-saving.