Epoxy resin (ER) composite coatings on carbon steel were prepared with polyaniline (PANI)-functionalized silicon nitride (Si3N4) powders (PANI/Si3N4) as fillers. First, a 5% mass ratio of Si3N4 fillers for ER coatings was selected to achieve a better physical and anticorrosion performance of Si3N4/ER coatings than that of ER coatings. Then, to attain PANI/Si3N4/ER coatings, PANI coated Si3N4 fillers were prepared and added into the ER matrix at 5% mass ratio. The adhesion strength was further improved, and the water absorption rate was decreased for PANI/Si3N4/ER coatings compared to Si3N4/ER coatings. In electrochemical impedance spectroscopy tests, the resistance values of the PANI/Si3N4/ER coatings were all higher than those of Si3N4/ER coatings and ER coatings. All results revealed the excellent barrier effects of PANI-functionalized Si3N4 fillers and the significant roles of PANI in the prevention of coating elimination caused by interface defect sites. Finally, an anticorrosion mechanism of PANI/Si3N4/ER coatings was proposed.