This paper applies spectral diagnostics (SD) to study the characteristics of diesel flames in opposed burners and monitor the operation of an entrained-flow opposed multi-burner (OMB) gasifier. On the basis of a bench-scale OMB gasifier, the spectral emissions of opposed impinging diesel flames were obtained for different planes and equivalence ratios. The emission lines of OH*, CH*, C-2*, Na*, Ar*, and K* radicals were found in diesel flame, and OH* was considered as a good indicator of the impinging flame height. The time-averaged and time-dependent OH* emission intensities can characterize the reaction zone and pulsation magnitude of the flame. The OH*, CH*, and C-2* emission intensities of two- and four-burner impinging flames under different [O/C]e were analyzed, indicating that the strengthening effects of four-burner impinging on chemical reactions are superior to those of two-burner impinging. Moreover, whether varying the O-2 velocity or diesel flow rate, the OH*/C-2* intensity ratio in the impinging region is the most suitable to estimate [O/C](e) for the OMB gasifier. The SD method was confirmed as a useful way to derive and monitor the flame temperature in an OMB gasifier.