The invention discloses a preparation method of a beta molecular sieve. The method comprises the following steps: step 1, synthesizing initial gel: mixing a silicon source, an aluminum source, an alkaline metal source and water, and performing uniform stirring to synthesize the initial gel, wherein based on SiO2 of the silicon source, Al2O3 of the aluminum source, and Na2O of the alkaline metal source, molar ratios of raw materials for synthesizing the initial gel are as follows: SiO2/Al2O3=(30-150):1, Na2O/SiO2=(0.03-0.35):1, and H2O/SiO2=(3.0-50.0):1; step 2, performing microwave treatment on the initial gel: adding a beta molecular sieve seed crystal into the initial gel prepared in the step 1, wherein a ratio of silicon to aluminum in the seed crystal is as follows: SiO2/Al2O3=(20-100):1, and the adding amount is 3%-25% by mass of SiO2, performing uniform mixing, and performing microwave heating treatment to obtain crystallized precursor gel; and step 3, performing a crystallization reaction: performing a crystallization reaction on the crystallized precursor gel obtained in the step 2 by microwave heating to obtain the beta molecular sieve product, wherein the reaction temperature is 140-180 DEG C, and the reaction time is 4-20 h. According to the method provided by the invention, the molecular sieve obtained by the method has a smaller particle size, a rich pore structureand a relatively-smooth channel, thereby facilitating promoting catalytic cracking and isomerization.