Chemiluminescence information of the excited state radicals is significant for flame spectral diagnostics of practical diffusion flames. According to OH*, CH*, and C-2* emissions, the effects of different conditions on the flame chemiluminescence and structure characteristics were discussed in this study. The results showed that, with the increase of burner-to-plate distance (L/D-e), the flame changes from envelope shape to conical shape, which affects the reaction region of a flame. The reaction region of an impinging flame could be divided into two parts: the jet core section near the nozzle exit and the impinging section near the flat plate. Due to the change of flow field, the size of the reaction region in impinging section increases first and then decreases with the increase of L/D-e. Moreover, the equivalence ratio [O/C](e) and L/D-e are the two factors influencing the size of the reaction region, and the length of a reaction region can be predicted by these two parameters.