The invention discloses a preparation method of polyaniline for an overcharge preventing coating material of a lithium ion battery. The preparation method comprises the following steps of (1), dissolving an inorganic acid or an organic acid in water, then adding activated carbon into an obtained first mixture, and uniformly dispersing the activated carbon; (2), cooling a material to below 2 DEG C, then adding a refrigerant and aniline into the material, and sufficiently agitating an obtained second mixture; (3), adding a primary oxidant into the second mixture, then intermittently adding ammonium persulfate into an obtained third mixture to carry out a polymerization reaction, carrying out suction filtration, and washing a filter cake by using deionized water; and (4), putting the filter cake into ammonia water mixed liquor, agitating and dedoping an obtained fourth mixture, carrying out the suction filtration, then washing an obtained product until pH (potential of Hydrogen) is 7 to 8, and carrying out vacuum drying on the washed product, so that the polyaniline is obtained. According to the preparation method of the polyaniline for the overcharge preventing coating material of the lithium ion battery, the steps are simple; the process is stable and controllable; the operability is high; a dual-oxidant system is adopted; the ammonium persulfate is intermittently added in the dual-oxidant system; further, the generation of a side reaction can be inhibited; the made and obtained polyaniline can be realized in the mutual conversion between an insulator and an electric conductor through doping or dedoping, and can be used for an overcharge preventing coating of the lithium ion battery; and the overcharge safety problem of a battery core is efficiency improved.