The invention belongs to the field of organic synthesis and discloses a preparation method of 3,6,9-triethyl-3,6,9-trimethyl-1,4,7-triperoxynonane. The preparation method mainly comprises peroxidatic reaction, condensation polymerization reaction and a blending refining process and particularly comprises the following steps of: stirring concentrated sulfuric acid used as a catalyst as well as hydrogen peroxide and methyl ethyl ketone used as raw materials at the temperature condition of 10-50 DEG C for reacting for 0.5-6.0h, then cooling to 20 DEG C, standing for layering, and retaining an organic layer, wherein the molar ratio of the methyl ethyl ketone to the hydrogen peroxide to the concentrated sulfuric acid is 3:(1-6):(0.3-3); neutralizing the reaction solution with aqueous alkali with the concentration of 10-50% in the organic layer, carrying out washing refining, removing byproducts, after the reaction, standing, cooling, separating the solution, drying, and filtering to obtain a pure target product; and adding a diluent to obtain a finished product. The preparation method disclosed by the invention is simple in reaction process, higher in yield, safe and environment-friendly.