By means of the "swelling-diffusion-interfacial polymerization method" (SDIPM), we successfully coated polyaniline (PANi) onto the positively charged polystyrene (PS) particles, which electrostatically repulse each other. After initially forming aniline-swollen PS particles, diffusion of the monomer toward the aqueous phase was controlled through a slow addition of hydrochloric acid, eventually leading to its polymerization on the particle surface. It is an unique, facile, and efficient approach based on raw substrate particles with cationic surface, in comparison with the previous efforts focusing on laborious surface modification or customized design of the substrate particles. The synthesized composite particles have been extensively characterized using scanning electron microscope, transmission electron microscope, Fourier transform infrared, Raman spectroscopy, and thermogravimetry. The resultant PS/PANi core/shell conductive composites possessed a uniform, intact PANi overlayer, and furthermore, their mophology can be well controlled by simply changing weight ratio of aniline/PS. The importance of the results just consists in the fact that the limitation of constructing an overlayer on similarly charged substrate particles should be overcome by adopting the unique SDIPM.