In our previous paper, the phenazine-like structure of the poly-o-phenylenediamine (PoPD) and its three steady redox states have been revealed mainly by using in situ resonance Raman spectroscopy, It has also been shown that the semi-oxidized state of PoPD is the most stable state of PoPD, while the totally-oxidized state of PoPD is chemically unstable and can exist only at certain electrode potentials. In the present work, the more detailed reaction mechanism of a PoPD film in strong acid solution has been studied by using in situ UV-vis subtractive reflectance spectroscopy. The semi-oxidized state and the totally-oxidized state of PoPD have electronic absorption bands around 300 nm, 430 nm, 500 nm and 300 nm, 450 nm, 530 nm, 735 nm respectively in the in situ steady state UV-vis subtractive reflectance spectra with respect to the reduced state of PoPD, which verifies once again that three redox states of PoPD exist in the redox process of PoPD. Moreover, the relative intensity between two oxidized states of PoPD at the maximum absorption wavelength (lambda(max) reveals that only about one third of the semi-oxidized state of PoPD can be oxidized to the totally-oxidized state of PoPD, The in situ resonance Raman spectra and the cyclic voltammograms of PoPD display the same quantitative relationship. New absorption bands were observed in the in situ time-resolved UV-vis subtractive reflectance spectra with appropriate time resolutions, which illustrate the dynamic structure changes of PoPD in its redox process. These intermediate states of PoPD are more unstable than its three redox states.