A precise opto-electronic system is used to collect the flame radiation spectra from visible to ultraviolet range during a gasoline engine combustion process. The characteristic spectra of intermediate combustion products such as CH. CN. C2. H2O are detected and their variation during the combustion are analyzed. The effects of excess air coefficient and cylinder pressure on these spectra are also studied. The experimental resultS show that the three different combustion phases in a gasoline engine have different combustion characteristic spectra: a large number of atoms, molecules, ions and radicals which are in their excited states are existing during the ignition process, with the development of the combustion, the spectra intensities of CH, C2 radicals increase rapidly. When the excess air coefficient is diminished, the spectra intensities decrease and the ignition delay which can be seen from the change of the spectra pattern becomes longer; The variation of the radical light intensity indicates the change of its concentration during the combustion. As the result, the flame emission spectroscopy is a powerful method to analyze and monitor the formation of poisonous exhausts, especially as CH, C2 and it can provide useful data for theoretical analysis, computor simulation of the combustion process and the controlling of the poisonous exhausts.