The redox process of poly-o-phenylenediamine (PoPD) film in strong acid solution has been studied by in situ resonance Raman spectroscopy, in situ UV-vis subtractive reflectance spectroscopy and cyclic voltammetry. A marked potential-dependent spectral change is observed in the in situ resonance Raman spectra, which reveals that at least three redox states of PoPD are formed in the redox process and the structure of PoPD is dominated by a ladder polymer with phenazine rings. The three kinds of PoPD display different resonance Raman enhancement effects, which is verified by the in situ UV-vis subtractive reflectance spectra. The ex situ resonance Raman spectra indicate that the electronic structure of the PoPD molecule is evidently influenced by protonation and that the most stable state of PoPD is its semi-oxidized state. Finally, a possible reaction scheme of the electrochromic process of PoPD film in strong acid solution is proposed.