The invention relates to the technical field of beta molecular sieves, and discloses a nanometer Sn-beta molecular sieve and a preparation method thereof and a phenol hydroxylation method. The preparation method comprises the following steps: (1) mixing a silicon source, a tin source, a structure-directing agent and an alkaline source in the presence of an aqueous solvent to obtain a mixed solution; adjusting the pH of the mixed solution to be 9.5-13.8; crystallizing the mixed solution; and separating and roasting to obtain a crystallized product; (2) mixing the crystallized product, cobalt salt and/or nickel salt based on ratio in the aqueous solvent to obtain a reacted solution; and enabling the reacted solution for 2-10 hours at the temperature of 180-220 DEG C to obtain a molecular sieve precursor; and (3) mixing the molecular sieve precursor and a sulfur-bearing solution in the aqueous solvent; and performing hydrothermal reaction to obtain the nanometer Sn-beta molecular sieve. The composite nanometer Sn-beta molecular sieve prepared by the above mentioned method is uniform in particle size, large in specific surface area, few in framework defects, and high in catalyzing performance in phenol hydroxylation.