The invention discloses a preparing method of an H-ZSM-34 molecular sieve. The preparing method of the H-ZSM-34 molecular sieve comprises the steps of preparing a mixture A from n(Al2O), n(Na2O), n(K2O), n (SiO2) and n (H2O) according to the proportion of 1:5.1:9.13:10.54:129.98, adding an aluminum source into an aqueous solution, continuing to add an alkaline source and a silicon source, and after stirring, ageing to obtain a clarified crystal seed solution; adding sodium hydroxide and water-soluble silica gel into an aqueous solution with a certain volume, after stirring, adding aluminum sulfate crystals and a crystal seed solution with a certain volume in sequence, stirring at the room temperature, putting the mixture into a kettle, conducting crystallization, taking the solution out ofthe kettle, conducting suction filtration and washing, and conducting drying and roasting to obtain Na-ZSM-34; adding a certain amount of Na-ZSM-34 into a 1mol / L ammonium nitrate solution according to the solid-to-liquid ratio of 1:20 to be stirred, and conducting suction filtration, washing and drying; repeating ammonia exchanging four times, afterwards, obtaining NH4-ZSM-34, and roasting NH4-ZSM-34 through air to obtain the H-ZSM-34 molecular sieve. The conversion rate of chloromethane in the reaction of using the H-ZSM-34 molecular sieve to prepare propylene through chloromethane is 87.0%,the selectivity sum of ethylene and propylene is 41.4%, the sum of the yields of ethylene and propylene is 36%, and the H-ZSM-34 molecular sieve does not get inactivated within 24 h. The preparing method has the advantages of being simple in technology, easy to operate and high in production efficiency.