This study presented a novel diagnosis for the combustion of coal-water slurry (CWS) in a pilot-scale slag-tap vertical cyclone furnace using flame images and three-dimensional temperature distributions reconstructed from these flame images through a two-step inverse radiation analysis. The accuracy of the reconstructed temperature was evaluated by infrared thermometer measurements, with a discrepancy of less than 50 K. The effects of the air/fuel equivalence ratio, the air distribution pattern, and the furnace load on combustion performance were analyzed based on the reconstructed temperature profiles. Experimental and analytical results showed that flame stability was clearly represented by the variance of the image pixel values. The three-dimensional temperature profiles also effectively illustrated the combustion characteristics in the cyclone furnace under different air/fuel settings. By appropriately supplying air, the combustion efficiency of CWS can be maintained at >99% in the proposed cyclone furnace.