The invention discloses an isoindole nitrogen oxide free radical modified polyaniline and a preparation method and an application thereof. 2-, 3-, 5- and 6-site side groups in aniline repeating structural units of a polyaniline high molecular material are chemically modified by an isoindole nitrogen oxide free radical derivative to obtain the product. Or 2-, 3-, 5- and 6-site side groups of aniline monomers are chemically modified by the isoindole nitrogen oxide free radical derivative, and then a polymerization reaction is carried out to obtain the product. Compared with polyaniline, the free radical polymer has better electrical conductivity, higher charge storage density, better capacitance retention rate, fast electrode reaction dynamics performance, and oxidation-reduction ability and characteristic three-line electron paramagnetic resonance spectra similar to isoindole free radicals; and the polymer has good rate capability and cycle stability in the processes of charging and discharging, and is expected to become a novel high-capacity and high-power battery material.