NOVELTY - Preparation of a nanometer solid alkali catalyst comprises adding a nanometer oxide particle having a particle size of less than 50 nm to ethanol or methanol, and stirring to form a suspension system; adding a dispersant, vibrating in an ultrasonic wave; and dissolving a soluble aluminum salt in the suspension system, adding dropwise calcium hydroxide aqueous solution until the pH is 6-8, continuously ultrasonic oscillating for greater than or equal to 30 minutes to obtain a suspending liquid, filtering, drying, grinding, and calcining.    USE - Method for the preparation of a nanometer solid alkali catalyst with a core-shell structure for synthesizing a biodiesel (claimed).    ADVANTAGE - The catalyst has good catalytic effect to high acidity oil for synthesizing the biodiesel. It simplifies the pre-treatment processing step of high acidity oil, and reduces the cost.    DETAILED DESCRIPTION - Preparation of a nanometer solid alkali catalyst with a core-shell structure comprises selecting a nanometer oxide particle having a particle size of less than 50 nm, adding to ethanol or methanol, and stirring to form a suspension system; adding a dispersant to the obtained suspension system, vibrating in an ultrasonic wave to be uniformly dispersed in the suspension system; and dissolving a soluble aluminum salt in the suspension system, adding dropwise calcium hydroxide aqueous solution until the pH is 6-8, continuously ultrasonic oscillating for greater than or equal to 30 minutes to obtain a suspending liquid, filtering, drying, grinding, and calcining. The mass ratio of the nanometer oxide particle, the soluble aluminum salt, and the dispersant is 1:0.2-0.8:0.1-0.3. The oxide particle comprises an oxide of magnesium, calcium, silicon, and/or aluminum. The soluble aluminum salt comprises aluminum sulfate, aluminum nitrate, or aluminum chloride. The dispersant comprises polyethylene glycol monomethyl ether, polyethylene glycol with a molecular weight of 2000-20000, sodium dodecyl sulfate, or sodium lauryl sulfate.