In order to modify hypocrellin A (HA) while maintaining its excellent sensitivity to singlet oxygen, work has been done to recognise its spectroscopy and electronic excited state properties. New evidences are obtained to decide its spectroscopy and electronic transition mechanism, from systemmatic study of the absorption and fluorescence spectrum of HA in different acidity and basicity. Results indicate that the absorption band I results from π→π* transition; the long wavelength absorption band Ⅱ and Ⅲ results from L→aπ transition for the reason of P-π conjugation, and the fluorescence emission band I and Ⅱ result from the transition of S1 (L,aπ)→S0 with the similar vibronic featrues to absorption band.