The drop tube furnace system was used to study the characteristics of ash deposition growth under different temperatures of probe surface in the mixed atmosphere of NH₃ and SO₃. The ash deposition process was recorded by the charge coupled device (CCD) camera, and the temperature inside and outside of the probe was monitored in real time to obtain the change of the thickness of ash deposition and the heat flux on the probe surface. And the ash deposition characteristics were further studied by the X-ray diffraction (XRD), scanning electron microscope (SEM), and energy dispersive spectrometer (EDS). Experimental results demonstrate that the stable thickness of the ash deposition decreases and the relative heat flux on the probe surface increases as the surface temperature of the probe increases. When the probe surface temperature was 200, 230 and 260 °C, the average ash deposition thickness was 2.15, 1.82 and 1.40 mm, respectively, and the relative change in average heat flux was 30.6%, 40.3% and 41.8%, respectively. The ash deposition growth can be divided into three stages, the rapid growth stage, the slow growth stage and the stable stage. The ammonium bisulfate (ABS) was detected by XRD, and the N element was detected by EDS, indicating that the generation of viscous ABS in a mixed atmosphere aggravates fly ash deposition. SEM results show that the ash deposition on the upper part of the probe is mostly agglomerated and the particle size increases, and the lower ash deposition is mostly spherical, similar to the fly ash. In addition, the agglomeration of the upper ash deposition is weakened, and the lower ash deposition becomes smoother and looser as the surface temperature of the probe increases.