The invention provides a method for preparing a high-hydrothermal-stability mesoporous molecular sieve. The method includes the following steps: (1) preparation of a first Y-type molecular sieve precursor; and (2) crystallization: adding a seed crystal to the first Y-type molecular sieve precursor, adjusting the pH value to 0.5-5, stirring for 10 h-24 h at the temperature of 20 DEG C-50 DEG C, then aging for 2 h-24 h at the temperature of 20 DEG C-50 DEG C, to obtain an assembled product, transferring the assembled product into a microreactor with a polytetrafluoroethylene lining, then transferring together with the reactor into a high-pressure reaction kettle, crystallizing for 10 h-48 h at the temperature of 100 DEG C-200 DEG C, filtering, washing, drying, and thus obtaining the high-stability mesoporous molecular sieve. Firstly, the microporous molecular sieve precursor is prepared, a mesoporous-microporous molecular sieve as the seed crystal is used for preparation of the mesoporous molecular sieve, the molecular sieve precursor assembly method and the seed crystal method are combined, and the high-stability mesoporous molecular sieve is obtained under the condition without use of an organic template agent. Not only is the preparation cost of the molecular sieve reduced greatly, but also the process of calcination of the template agent is omitted so as to reduce the energy consumption.