In situ UV-Vis spectroelectrochemical spectra of the electrochemical copolymerization of aniline(AN) and o aminobenzenesulfonic acid(ABSA) was investigated in 01 mol/L HClO4.The homopolymerization of ABSA and AN also arised independently in the same medium.The results indicated that ABSA was firstly oxidized to its cation radical，then a mixed oligomer intermediate was formed by the cross-reaction of ABSA and AN cation radicals with their monomers.The absorption peak at 550 nm in the UV-Vis spectra was assigned to this intermediate.The intermediates then reacted with each other to form the polymer finally.The results also indicated that the polymerization of AN became dominant with the increase of the AN amount in the mixed solutions,which demonstrated that the copolymerization mechanism was related to the concentration ratio of ABSA and AN.Fourier transform infrared spectra also indicated the effective attachment of ABSA on the PAN backbone formed the self doped PAN.