The present invention provides a preparation method of Beta molecular sieve, comprising: activating a mineral having low silica-to-alumina ratio and a mineral having high silica-to-alumina ratio, respectively, wherein the mineral having low silica-to-alumina ratio is activated via a sub-molten salt medium, and the mineral having high silica-to-alumina ratio is activated via means of high-temperature calcination; mixing the activated minerals with sodium chloride, potassium chloride, water and template agent for hydrothermal crystallization, wherein the charged amounts of the raw materials satisfies a molar ratio of: 0.03-0.18 Na2O: 0.01-0.03 K2O: 0.1-0.4 (TEA)2O: 1 SiO2: 0.01-0.5 Al2O3: 12-40 H2O; cooling the crystallized product and removing the mother liquor by filtration, washing the resulting filter cake with water to neutral and drying it to obtain the Beta molecular sieve.