The invention discloses a method for synthesizing an ultrathin ZSM-5 molecular sieve according to a low-temperature and normal-pressure hydrothermal method. Trace ammonium salt, silicon sol, a fluorine source and an aluminum source which serve as raw materials are subjected to hydrothermal synthesis. The method is characterized by comprising the following steps: stirring a template agent (ammoniumsalt), a silicon source and a certain amount of water uniformly at room temperature, sequentially adding the aluminum source and the fluorine source, and stirring and performing pre-crystallization at 50 to 100 DEG C for 1 to 50 hours; performing crystallization at 50 to 100 DEG C for 12 to 240 hours; after the reaction ends, cooling a reaction kettle to room temperature and then centrifuging, washing and drying the products. According to the method, the synthesis temperature is low, the synthesis pressure is low or normal pressure, the use amount of the template agent is small, skeleton heteroatoms can be regulated and controlled, and the thickness of a nanometer sheet can be regulated and controlled effectively. The crystallinity degree of the ultrathin nanometer sheet-like ZSM-5 molecular sieve is high, the synthesis cost is greatly saved due to a trace amount of template agent, the use of a high-pressure reaction kettle can be avoided due to low temperature and normal pressure, and the method is favorable for industrial amplification production and has strong industrial promotion prospect.