In an opposed multi-burner (OMB) gasifier, fuel is injected through four burners located approximately one-third down the length of the reactor and the flames impinge on each other in the center of the reactor. Understanding the impingement and flame interaction is important for system design, operation, and troubleshooting. On the basis of the bench-scale opposed multi-burner gasifier, industrial cameras combined with high-temperature endoscopes and image processing technology are applied to study the characteristics of the impinging flame height of diesel and coal-water slurry (CWS) gasification. A new approach for flame image segmentation based on the pseudo-color enhancement is proposed. The results show that both the impinging flame heights of diesel and CWS increase with the increase of mole ratio of oxygen/carbon (O/C), while a turning point appears at O/C of 1.8 when it is diesel gasification. In comparison to diesel, the amplitude of the increase of CWS flame heights is smaller. The increase of gasification capacity results in the decrease of the fluctuation range of impinging flame heights. The results of fast Fourier transform (FFT) of instantaneous impinging flame heights show that the major frequencies of the impinging flame height are below 10 Hz for both diesel and CWS and the amplitudes of characteristic peaks decrease with the increase of O/C. The impinging flame heights of CWS fluctuate in a larger range compared to diesel.