The problem of ammonium bisulfate (ABS) deposition blocking and corroding air preheaters and economizers in boilers became severe, as the selective catalyst reduction (SCR) system was widely used in the coal-fired boilers to meet the stringent nitrogen oxide (NOx) discharge standard. In this study, an online digital image technique was applied to investigate the growth of the deposition of fly ash and ABS blend in a drop tube furnace (DTF). The deposition was measured on a temperature controlled deposition probe. The results of the image technique showed that the surface temperature of the deposition probe had a negative effect on stable thicknesses of the blend depositions and the stable thicknesses for cases 1 (553 K), 2 (513 K), and 3 (473 K) are 4.78, 5.26, and 7.64 mm, respectively. Meanwhile, the relative variation of the heat flux Q(R) for Cases 1, 2, and 3 are 17.8%, 33.8%, and 40.2%. The deposition in the lower deposition surface temperature has a greater effect on the heat transfer efficiency. In order to understand the deposition formation of fly ash and ABS blend, the X-ray diffraction (XRD), X-ray fluorescence (XRF), and the scanning electron microscopy (SEM) analysis were undertaken of the blend deposition. The results indicated that the content of ABS in the bottom part deposition increased as the probe surface temperature decreases, and this would result in more agglomerates in the deposition according to the observations of the SEM and the growth behavior of the deposition was influenced. Meanwhile, the results show that the deposition characteristics of ABS and fly ash blend is strongly influenced by the distribution and the condensation behavior of ABS. This work makes a contribution to a deeper understanding of deposition characteristics of ammonium bisulfate and fly ash blend, which is important for boilers to control the blend deposition in air preheaters and economizers.