The invention provides a preparation method of a nano-filtration membrane. The preparation method comprises an interfacial polymerization reaction step and a post-processing step, wherein the interfacial polymerization reaction step is as follows: preparing an aqueous phase solution and an oil phase solution, pouring the aqueous phase solution and the oil phase solution into the membrane surface of the nano-filtration membrane in sequence and pouring away; the post-processing step is as follows: evaporating for 0.2-2 minutes in a baking oven with a temperature of 80-100 DEG C, then immediately soaking in an aqueous solution containing 30-100 weight percent of modifying agent for 30-120 seconds after evaporation and preparing a reverse osmosis membrane. As the membrane obtained after the completion of interfacial polymerization reaction is soaked in the aqueous solution containing the modifying agent in the preparation process, the high-performance nano-filtration membrane of which the throughout and the organic small molecular interception rate are obviously improved is obtained.