The invention discloses a preparation method of a mesoporous molecular sieve, belonging to the technical field of porous material preparation. The preparation method comprises the following steps: (1) adding deionized water and acid or alkali into a flotation cell of a flotation machine, and stirring; (2) adding a template agent into the flotation cell, stirring, starting an inlet valve of the flotation machine, and when bubbles float, starting a scraper blade to scrape the bubbles; (3) slowly bringing a silicon source into the flotation cell by a gas inlet pipe along with negative pressure of gas, stirring until sol is formed, closing the inlet valve after 30-60min, and transferring into a teflon reaction kettle; (4) putting the reaction kettle into a drying oven, crystallizing for 24-72h under 100 DEG C, then taking out, washing, filtering and drying a product to obtain a solid product, and calcinating for 4-6h under 500-600 DEG C to obtain a mesoporous molecular sieve material. The mesoporous molecular sieve product prepared by the method is high in pore-forming rate and uniform in pore diameter; the method is wide in use scope, and has very good application prospect.