Information on luminescence of the excited-state radicals is of significance for spectrometer-based diagnostics of practical diffusion flames. In this paper, the emission spectrometry was applied to detect the distribution characteristics of three excited-state radicals (OH*, CH*, and C-2*) in CH4/O-2 co-flow jet diffusion flames with relatively high flow velocities (66 m/s CH4 and 18 m/s O-2/N-2), which is the basis to rationalize chemiluminescence flame diagnostics in gasification and combustion processes. This method allowed for the online measurement of luminescence intensity in flames. Results showed that there exited two peaks along the flame propagation direction for OH* when [O/C](e) was greater than 0.90. The first peak formed near the nozzle exit because of the relatively high flow velocities pushing the interdiffusion of fuel and oxygen in the flame bottom, and the second peak was located near the boundary region of blue and yellow zones, indicating where the other reaction zone is. In contrast, CH* and C-2* demonstrated only one chemiluminecent peak near the nozzle exit. OH*/CH* intensity ratio exhibited an exponential correlation with [O/C](e) in the blue region, where the background spectra could be negligible, which can be regarded as the same as those found in premixed flames, and it is valuable to characterize the local [O/C](e) quantitatively.