In this paper, the drop tube furnace (DTF) system was utilized to study the ash deposition growth of different temperatures in the mixed atmosphere of NH3 and SO3 using an on-line digital image technique. A series of tests, such as XRD, SEM, and EDS, were also used to further study the ash deposition characteristics. The experimental results show that the stable thickness of ash deposition decreases and heat flux at the probe surface increases as the temperature increases. The average ash deposition thickness at the temperatures of 553, 593 and 633 K is 2.13, 1.64 and 1.53 mm, respectively, and the average heat flux is 17.13, 24.19 and 31.33 kW/m(2), respectively. Ash deposition growth can be divided into three stages: the rapid growth stage, the slow growth stage, and the stable stage. Ammonium bisulfate (ABS) was detected by XRD, and an N element was detected by EDS, indicating the viscous ABS generated in the mixed atmosphere and aggravated fly ash deposition. SEM analysis showed that the ash deposition on the upper part of the probe was mostly agglomerated and the particle size increased, 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 temperature increases. (C) 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.