Abstract: The firetube waste heat boiler in the deoiled asphalt gasification process has serious scaling problems, which not only affect the heat transfer efficiency of the firetube boiler, but also threaten the safe operation of the boiler. In view of the special ash composition and large temperature difference between inlet and outlet in deoiled asphalt, combined with the melting liquid phase ratio model, critical velocity model and shedding model, the CFD method is used to segmentally simulate the ash deposition distribution, depositional surface temperature and outlet temperature changes in industrial firetube boilers, and explore the related impact of syngas inlet flow. The entire firetube is divided into seven segments A—G. The results showed that the ash deposition in the firetube boiler mainly concentrated in segments A and B, with a maximum depositional thickness of 6.37 mm. After ash deposition, the overall heat transfer efficiency only decreased by 0.88%, but the heat transfer efficiency of each tube section changed significantly. When investigating the influence of syngas flow, it was found that the syngas outlet temperature was positively correlated with the syngas flow. When the syngas flow (Taking the operating condition of 28.9 m/s as the benchmark, the inlet gas velocity was set at 80%, 90%, 100%, 110%, and 120% of the operating condition. Throughout the text, this represented the syngas flow) increased from 80% to 120%, the outlet temperature at the end of the firetube G section only increased from 590.51 K to 605.56 K. Increasing the syngas flow could be adopted to enhance the steam output. However, as the syngas flow increased, except for the wall temperature of section A decreasing, the wall temperatures of sections B to G all increased accordingly. Under the condition of a syngas flow of 120%, the wall temperatures of the highest-temperature sections B and C reached 756 K and 768 K, respectively, approaching the maximum working temperature of the material 773 K, which was not conducive to the stability of the boiler operation.