Abstract: Using three different length chain alkyl quaternary ammonium cationic surfactants which are dodecyl trimethyl ammonium bromide (DTAB) , tetradecyl trimethyl ammonium bromide (TTAB) and cetrimonium bromide(CTAB) to modify the fly ash floating beads (FAFB).The polyaniline @organic modified fly ash floating beads (PAn@OFAFB) composites were obtained by emulsion polymerization using modified products and aniline(An). The structure characterization and electrorheological property of composites were analyzed by chemical element analysis, FTIR, XRD, SEM, TGA, four point probe technique , LCR dielectric spectrometer and rotational viscometer. The results show that the longer the carbon chain length of modified agents, the better modification effect, and the surface of FAFB modified with CTAB obtains the maximum adsorption amount, surface adsorption ratio is 1.91wt%, the PAn and OFAFB compound better and FAFB surface is surrounded by more PAn. The highest conductivity is 0.06 S·cm^-1 which obtained by polyaniline @ cetrimonium bromide modified fly ash floating beads (PAn@FAFB-CTAB).The electrorheological effect of electrorheological fluid with PAn@FAFB-CTAB as dispersed phase is better than PAn-dodecylbenzene sulfonic acid (PAn-DBSA) and the breakdown capacity increases by 0.5 kV/mm compared with pure PAn. The suspension stability of composites can be improved by making FAFB as the carrier of PAn.The thermal stability of composites is enhanced significantly owing to the introduction of FAFB-CTAB, and the relative amount of PAn coated is 28.07wt%.